Programmable high-speed cable with boost device

ABSTRACT

An HDMI cable carries high speed encoded data which are transmitted differentially over data channels, along with a clock. High-frequency loss and differential skew within a differential signal may be compensated by analog circuits embedded in the cable. These embedded circuits are tuned at production for best performance by observing the quality of the recovered analog signal. The embedded circuits are powered by a combination of power sources, both carried within the cable, and harvested from the high-speed signals themselves.

FIELD OF THE INVENTION

The present invention relates to high speed cables that carry seriallyencoded differential signals between electronic equipments, and inparticular, multi-conductor cables interconnecting audio-visualequipment.

BACKGROUND OF THE INVENTION

The distribution of television signals has increasingly become based ondigital methods and digitally encoded forms of video and audio signals.At the same time, higher resolution (high definition TV) has becomeavailable in the market place, commensurate with larger and higherdefinition displays. To meet the requirement of interconnecting suchhigh definition displays with digital signal sources such as DigitalVersatile Disc (DVD) players and receivers/decoders for digitalsatellite and digital cable distribution of video material, a digitalinterface standard has evolved, known as the High-Definition MultimediaInterface (HDMI). A detailed specification for HDMI can be obtained fromthe “hdmi.org” website. The HDMI specification currently available andused in this application is HDMI specification version 1.3 dated Jun.22, 2006, which is incorporated herein by reference. This HDMI standardcan be employed for connecting digital video sources to digital videosinks over a cable that carries a number of digital signals and a clocksignal.

The inherent characteristics and manufacturing imperfections ofhigh-speed differential signaling cables such as may be used to carryHDMI signals have an adverse effect on the high-speed signals carried bythe cable.

For example, any cable has a limited bandwidth and therefore acts as alow pass filter. The bandwidth of the cable is related to its length,the longer the cable the greater the filtering effect and the lower itsbandwidth. As a result, high-frequency signals passing through the cableare attenuated, and their edges become less sharp. This leads to anincreased risk of misinterpreting the received data at the receiver endof the cable, especially for long cables and high-speed data.

FIGS. 1A-1C illustrate the effect of the limited bandwidth of a cable onthe transmitted signals. FIG. 1A illustrates a high-speed signal to betransmitted through a high-speed cable, FIG. 1B shows a distortedbandwidth-limited signal received at the receiver end of the cable(before equalization), and FIG. 1C shows the received signal at thereceiver end after equalization. As seen from FIG. 1B, the signal edgesare slowed and short pulses are narrowed, not reaching the fulltransmitted amplitude.

Differential signaling cables are commonly used to carry high-speeddigital signals in differential form, that is pulses of opposingpolarities are transmitted on the two strands of the cable. Thedifferential signal carried over such cables may be warped, that is thetwo signal components (positive and negative polarities V+ and V−) areskewed in time with respect to each other (differential skew), furtherdistorting the received signal.

The impact of differential skew is depicted in timing diagrams in FIGS.2A and 2B.

FIG. 2A shows an example timing diagram of the two single ended signalcomponents (V+, V−) of the differential data on an HDMI channel, as itmay be transmitted by an HDMI source into a cable. A timing diagram ofthe corresponding differential signal (Vdiff−xmit) in FIG. 2Aillustrates the corresponding differential signal that is clean andeasily interpreted.

FIG. 2B shows an example timing diagram of the two single ended signalcomponents (V+ and V-del) of the differential data on an HDMI channel,as it might be received at the end of a cable. For the sake of clarity,only the effect of the differential skew is shown in FIG. 2B. Thesignals V+ and V− are skewed in time with respect to each other. Thenegative signal component V− is delayed with respect to the signalcomponent V+ by a differential skew delay of Td. A timing diagram of thecorresponding distorted differential signal (Vdiff−rcv) in FIG. 2Billustrates that, as a consequence of the differential skew, thedifferential signal Vdiff−rcv is significantly distorted with clearlyvisible plateaus in the signal where the differential signal is zero(0). These plateau regions can only be interpreted as noise by thereceiver, the result of which is to reduce the width of the window ofvalid data. This reduction is seen as a closure of the receive data eyeand directly compromises the channel quality. The amount of differentialskew delay (Td) depends on the characteristics of each individual cable,and is basically constant.

Earlier approaches to improving cable quality so far have been limitedto embedded passive equalizer circuits within the cable, which boosthigh frequencies of the signals attenuated in the cable. Such equalizersare fixed to compensate for a fixed cable length.

While the equalization required for a given cable depends largely on thelength of the cable, other characteristics of high-speed signalingcables such as the differential skew, being more random, may varysubstantially between the cables.

Accordingly, there is a need in the industry for the development of animproved high-speed signaling cable, which would provide improved signalcharacteristics.

Earlier High-Definition Multimedia Interface (HDMI) signal boosters thatcan be used to boost HDMI signals use external power inputs, see e.g.Long Reach™ product of Gennum corporation, which can be found atwww.gennum.com/ip/pdffiles/gs8101.pdf. As a result, they cannot beembedded in a standard HDMI cable. A more recent development is astand-alone “super booster” that can be inserted inline with a cable,and is also available integrated in an HDMI cable, see references: GefenInc., http://www.gefen.com/kvm/productjsp?prod_id=2939, including anadvertisement of a standalone HDMI “super booster; A manual for thestandalone HDMI “super booster, which can be found athttp://www.gefen.com/pdf/EXT-HDMI-141SB.pdf; and an advertisement for acable with an integrated HDMI “super booster”http://www.gefen.com/kvm/cables/hdmicables.jsp#hdmisb

The possibility of embedding an active device within the cable isassociated with a problem. Firstly, no power input may be available forsuch a device except through the cable, i.e. there is no provision forexternal power supplies. Secondly, in the case of the HDMI cable, thereis not enough power available to power a simple signal regenerator,primarily because of the specification requirement to provide atermination voltage for the inputs. As a result, the embedded activedevice apparently cannot be powered as required.

In more detail, the main power requirement for an HDMI signal booster isthe requirement to provide a termination voltage (3.3V) with thecapability to source 12 mA for each of three HDMI inputs. The power thatis available from the cable comes from a 5V line, from which a maximumcurrent of 5 mA can be drawn (as per HDMI specification V 1.3) when thesink device is active, i.e. the total available power is limited to 50mW. The combined power requirement of the input terminations on theother hand is approximately 12 mA*3.3V*3=120 mW. Unfortunately, theserequirements cannot be met in a standard HDMI cable in a simple way.

Accordingly, there is a need in the industry for the development of animproved signal booster with an improved power control circuit forembedded cable applications based on or more active devices, which wouldavoid or mitigate the above noted problem.

SUMMARY OF THE INVENTION

There is an object of the invention to provide an improved programmablecable with embedded power control and boost device as well as methodsand systems for calibrating the cable.

According to one aspect of the invention there is provided a cable forconnecting a transmitting data source device to a receiving data sinkdevice and carrying differential data signals including: a boost devicefor boosting at least one of the differential data signals, the boostdevice comprising: an electronic circuit for obtaining at least some ofthe electrical power required to operate the boost device from the atleast one of the differential data signals.

The differential data signals are differential High DefinitionMulti-Media Interface (HDMI) signals and include a plurality ofTransition Minimized Differential Signaling (TMDS) encoded data channelsand a clock channel.

The boost device includes: a differential input circuit for receivingone of the differential data signals from the data source device; and adifferential output circuit for transmitting a boosted one of thedifferential data signals to the data sink device, wherein said at leastsome of the electrical power is obtained from the data source and sinkdevices.

The differential output circuit and the differential input circuit areconnected in series so as to conduct a current from the data sink deviceto the data source device.

The differential output circuit and the differential input circuit arejoined at an intermediate voltage node such that load current from thedata sink device flows through the differential output circuit to theintermediate voltage node, and the intermediate voltage node isconnected as the supply voltage for the differential input circuit.

The cable further includes a voltage boost circuit between theintermediate voltage node and a second intermediate voltage nodesupplying voltage for the differential input circuit.

The voltage boost circuit includes a switched capacitor and a 2-phaseclock, the capacitors used for periodically transferring energy from theintermediate voltage node to the second intermediate voltage node.

The boost device further includes a processing block having a transferfunction for processing the differential signal received by thedifferential input circuit and conveying the processed signal to thedifferential output circuit.

The cable further includes a power converter for converting the powerfor operating the processing block from an available higher voltage, thepower converter comprising switched capacitors and a 2-phase clock, theswitched capacitors used for transferring energy from the availablehigher voltage to the processing block.

According to another aspect of the invention, there is provided a methodfor providing power to a boost device in a cable connected between atransmitting data source device and a receiving data sink device,comprising the steps of: receiving differential data signals from thedata source device in a differential input circuit of the boost device;boosting at least one of the received differential data signals into aboosted differential data signal; transmitting the boosted differentialdata signal to the receiving data sink device with a differential outputcircuit of the boost device; and obtaining power to operate at leastsome of the circuitry of the boost device from the data source and sinkdevices through their connections with the differential input and outputcircuits respectively.

The method further comprises the step of connecting the differentialoutput circuit and the differential input circuit in series so as toconduct a current from the data sink device to the data source device.

The method as described above further comprises the steps of: joiningthe differential output circuit and the differential input circuit at anintermediate voltage node such that load current from the data sinkdevice flows through the differential output circuit to the intermediatevoltage node; and connecting the intermediate voltage node as the supplyvoltage for the differential input circuit.

According to yet another aspect of the invention there is provided aboost device for connecting a transmitting data source device to areceiving data sink device, the transmitting data source device sendingdifferential data signals into the boost device, the boost device forboosting at least one of the differential data signals, the boost devicecomprising an electronic circuit which obtains at least some of theelectrical power required to operate the boost device from the at leastone of the differential data signals.

In the boost device as described above, the differential data signalsare differential High Definition Multi-Media Interface (HDMI) signalsand include a plurality of Transition Minimized Differential Signaling(TMDS) encoded data channels and a clock channel.

The boost device further comprises: a differential input circuit forreceiving one of the differential data signals from the data sourcedevice; and a differential output circuit for transmitting a boosted oneof the differential data signals to the data sink device; wherein saidat least some of the electrical power to operate the boost device isobtained from the data source and sink devices.

In the boost device described above, the differential output circuit andthe differential input circuit are connected in series so as to conducta current from the data sink device to the data source device.

The differential output circuit and the differential input circuit arejoined at an intermediate voltage node such that load current from thedata sink device flows through the differential output circuit to theintermediate voltage node, and the intermediate voltage node isconnected as the supply voltage for the differential input circuit.

The boost device described above further includes a voltage boostcircuit between the intermediate voltage node and a second intermediatevoltage node supplying voltage for the differential input circuit.

The voltage boost circuit includes a switched capacitor and a 2-phaseclock, the capacitor used for periodically transferring energy from theintermediate voltage node to the second intermediate voltage node.

The boost device further comprises a processing block having a transferfunction for processing the differential signal received by thedifferential input circuit and conveying the processed signal to thedifferential output circuit.

The boost device further comprises a power converter for converting thepower for operating the processing block from an available highervoltage, the power converter comprising switched capacitors and a2-phase clock, the switched capacitors used for transferring energy fromthe available higher voltage to the processing block.

According to one more aspects of the invention there is provided a cablefor connecting a transmitting data source device to a receiving datasink device carrying differential signals including: a boost device forboosting at least one of the differential signals, the boost devicecomprising: an input circuit for receiving a raw differential signalfrom the data source device and outputting a recovered signal; a deskewcircuit with first adjustable parameters for processing the recoveredsignal into a deskewed signal; an equalizer circuit with secondadjustable parameters for processing the deskewed signal into anequalized signal; and an output circuit for amplifying the equalizedsignal into a boosted signal and sending the boosted signal to the datasink device.

In the cable described above, the boost device further includes aparameter memory for retaining the first and second adjustableparameters after they have been adjusted.

The cable further includes a control bus, and the parameter memory isaccessible from said control bus.

The equalizer circuit comprises a circuit for adjusting a frequencyresponse of the deskewed signal by changing the second adjustableparameters to produce the equalized signal. Preferably, the equalizercircuit has at least two settings of the second adjustable parametersfor adjusting the frequency response.

In the cable of the embodiments of the invention, the deskew circuit isan analog differential deskew circuit for adjusting an existing timeskew of two polarities of the differential signal by changing the firstadjustable parameters.

The analog differential deskew circuit comprises: a number of delayunits arranged sequentially; an analog selector, selecting a compositedelay resulting from the delay units that are selected by the analogselector; and analog switches inserting the composite delay into thepolarities of the differential signal.

The analog switches are inserting the composite delay into one or theother polarity of the differential signal. Preferably, each of theanalog delay units has a gain, which is substantially equal to 1.0, andeach of the analog delay units comprises one or more amplifiers. In moredetail, each analog delay unit comprises: first and second amplifiershaving a common input, which is the input of the analog delay unit, andtheir outputs being summed to generate the output of the analog delayunit; the first amplifier having a gain of (1.0−Δ), and a delay equal toa predetermined delay value; and the second amplifier having a gain ofΔ, and substantially the same delay as the first amplifier.Conveniently, the first amplifier is a follower stage, and e secondamplifier has a shunt capacitor for setting the gain of Δ.

According to one more aspect of the invention there is provided a boostdevice for connecting a transmitting data source device to a receivingdata sink device, the transmitting data source device sendingdifferential data signals into the boost device, the boost device forboosting at least one of the differential data signals, the boost devicecomprising: an input circuit for receiving a raw differential signalfrom the data source device and outputting a recovered signal; a deskewcircuit with first adjustable parameters for processing the recoveredsignal into a deskewed signal; an equalizer circuit with secondadjustable parameters for processing the deskewed signal into anequalized signal; and an output circuit for amplifying the equalizedsignal into a boosted signal and sending the boosted signal to the datasink device.

The boost device further includes a parameter memory for retaining thefirst and second adjustable parameters. The boost device also includes acontrol input for accessing the parameter memory.

The equalizer circuit comprises a circuit for adjusting a frequencyresponse of the deskewed signal by changing the second adjustableparameters to produce the equalized signal. The equalizer circuit has atleast two settings of the second adjustable parameters for adjusting thefrequency response.

The deskew circuit is an analog differential deskew circuit foradjusting an existing time skew of two polarities of the differentialsignal by changing the first adjustable parameters.

Preferably, the analog differential deskew circuit comprises: a numberof delay units arranged sequentially; an analog selector, selecting acomposite delay resulting from the delay units that are selected by theanalog selector; and analog switches inserting the composite delay intothe polarities of the differential signal. Advantageously, the analogswitches are inserting the composite delay into one or the otherpolarity of the differential signal.

Preferably, each of the analog delay units has a gain, which issubstantially equal to 1.0 and comprises one or more amplifiers. In theembodiments of the invention, each analog delay unit comprises: firstand second amplifiers having a common input, which is the input of theanalog delay unit, and their outputs being summed to generate the outputof the analog delay unit; the first amplifier having a gain of (1.0−Δ),and a delay equal to a predetermined delay value; and the secondamplifier having a gain of Δ, and substantially the same delay as thefirst amplifier.

Conveniently, the first amplifier is a follower stage, and the secondamplifier has a shunt capacitor for setting the gain of Δ.

According to yet one more aspect of the invention, there is provided amethod for a sending differential signals from a transmitting datasource device to a receiving data sink device through a cable thatincludes a boost device for boosting at least one of the differentialsignals, comprising the steps of: receiving a raw differential signalfrom the data source device in an input circuit of the boost device andoutputting a recovered signal; processing the recovered signal in adeskew circuit with first adjustable parameters into a deskewed signal;processing the deskewed signal in an equalizer circuit with secondadjustable parameters into an equalized signal; amplifying the equalizedsignal in an output circuit into a boosted signal; and sending theboosted signal to the data sink device.

The method further includes the steps of adjusting the first and secondadjustable parameters; storing the first and second adjustableparameters in parameter memory; and accessing the parameter memory froma control input. Conveniently, the step of processing the recoveredsignal includes the step of adjusting an existing time skew of twopolarities of the differential signal by changing the first adjustableparameters; and adjusting a frequency response of the deskewed signal bychanging the second adjustable parameters.

In more detail, the step of changing the first adjustable parameterscomprises the steps of arranging a number of delay units sequentially;selecting a composite delay resulting from number of delay units; andinserting the composite delay into the polarities of the differentialsignal. Beneficially, the step of inserting the composite delay includesinserting the composite delay into one or the other polarity of thedifferential signal. Conveniently, the step of arranging a number ofdelay units includes a step of selecting analog delay units each havinga gain that is substantially equal to 1.0.

According to one more aspect of the invention, there is provided a cablefor connecting a transmitting data source device to a receiving datasink device carrying differential signals including: a printed circuitboard (PCB) and a boost device, the PCB including tracks for providingdelays in coupling a raw differential signal from the data source deviceto two or more inputs of the boost device; the boost device for boostingat least one of the differential signals, the boost device comprising:an input circuit for terminating the delayed raw differential signal; aninput selector circuit with first adjustable parameters for selecting adelayed raw differential signal and outputting a recovered signal thatis deskewed; an equalizer circuit with second adjustable parameters forprocessing the recovered signal into an equalized signal; and an outputcircuit for amplifying the equalized signal into a boosted signal andsending the boosted signal to the data sink device.

The boost device further includes a parameter memory for retaining thefirst and second adjustable parameters. The cable also includes acontrol bus, and the parameter memory is accessible from said controlbus.

In the boost device, the input selector circuit for selecting thedelayed raw differential signal for adjusting an existing time skew oftwo polarities of the differential signal is controlled by changing thefirst adjustable parameters. The equalizer circuit comprises a circuitfor adjusting a frequency response of the deskewed signal by changingthe second adjustable parameters to produce the equalized signal.Conveniently, the equalizer circuit has at least two settings of thesecond adjustable parameters for adjusting the frequency response.

The PCB comprises a number of tracks providing delays arrangedsequentially, and the input selector circuit selecting a composite delayresulting from the tracks that are selected by the input selectorcircuit.

According to one more aspect of the invention, there is provided a cablefor connecting a transmitting data source device to a receiving datasink device carrying differential signals including: a printed circuitboard (PCB) and a boost device, the PCB including tracks for providingdelays in coupling a raw differential signal from the data source deviceto two or more inputs of the boost device; the boost device for boostingat least one of the differential signals, the boost device comprising:an input circuit for terminating the delayed raw differential signal; aninput selector circuit with first adjustable parameters for selecting adelayed raw differential signal and outputting a recovered signal thatis coarsely deskewed; a deskew circuit with second adjustable parametersfor processing the recovered and coarsely deskewed signal into a finelydeskewed signal; an equalizer circuit with third adjustable parametersfor processing the finely deskewed signal into an equalized signal; andan output circuit for amplifying the equalized signal into a boostedsignal and sending the boosted signal to the data sink device.

Similar to previous embodiments of the invention, the boost deviceincludes a parameter memory for retaining the first, second, and thirdadjustable parameters. The cable further includes a control bus, and theparameter memory is accessible from said control bus.

The input selector circuit for selecting the delayed raw differentialsignal for coarsely adjusting an existing time skew of two polarities ofthe differential signal is controlled by changing the first adjustableparameters, and in which the deskew circuit is an analog differentialdeskew circuit for finely adjusting a remaining time skew of twopolarities of the differential signal by changing the second adjustableparameters.

The equalizer circuit comprises a circuit for adjusting a frequencyresponse of the deskewed signal by changing the third adjustableparameters to produce the equalized signal. The equalizer circuit has atleast two settings of the third adjustable parameters for adjusting thefrequency response.

The cable as described above, wherein the PCB comprises a number oftracks providing delays arranged sequentially, and the input selectorcircuit selecting a composite delay resulting from the tracks that areselected by the input selector circuit, and wherein further the analogdifferential deskew circuit comprises: a number of delay units arrangedsequentially; an analog selector, selecting a composite delay resultingfrom the delay units that are selected by the analog selector; andanalog switches inserting the composite delay into the polarities of thedifferential signal. Beneficially, the analog switches are inserting thecomposite delay into one or the other polarity of the differentialsignal. Similar to other embodiments described above, each of the analogdelay units has a gain, which is substantially equal to 1.0 andcomprises one or more amplifiers.

Each analog delay unit comprises: first and second amplifiers having acommon input, which is the input of the analog delay unit, and theiroutputs being summed to generate the output of the analog delay unit;the first amplifier having a gain of (1.0−Δ), and a delay equal to apredetermined delay value; and the second amplifier having a gain of Δ,and substantially the same delay as the first amplifier. Conveniently,the first amplifier is a follower stage, and the second amplifier has ashunt capacitor for setting the gain of Δ.

According to yet one more aspect of the invention, there is provided acable for connecting a transmitting data source device to a receivingdata sink device carrying differential signals including: a boost devicefor boosting at least one of the differential signals, the boost devicecomprising: an input circuit for receiving a raw differential signalfrom the data source device and outputting a recovered signal; a deskewcircuit with adjustable parameters for processing the recovered signalinto a deskewed signal; and an output circuit for amplifying thedeskewed signal into a boosted signal and sending the boosted signal tothe data sink device.

The boost circuit further includes an equalizer circuit for adjustingthe frequency response of the deskewed signal.

The boost device also includes a parameter memory for retaining theadjustable parameters. The cable further includes a control bus, and theparameter memory is accessible from said control bus.

In this embodiment of the invention, the boost device further includesperformance analysis circuitry for determining the performance of thecable.

The performance analysis circuitry includes: a differential tosingle-ended block for converting the boosted signal to a single-endedsignal; a linear phase compensator to phase-align the single-endedsignal with a common clock signal; an oversampling circuit providing adigital representation of the phase-aligned single ended signal (apreprocessed data signal); and a training function circuit forestimating a quality of the preprocessed data signal, and adjusting theparameters of the deskew and equalizer circuitry (by changing theadjustable parameters) to improve the quality of the preprocessed datasignal.

The training function circuit further comprises: a digital circuit forestimating the quality of the preprocessed data signal and generating aQuality Number indicating said quality; an evaluation run controlcircuit for adjusting the parameters of the deskew and equalizercircuitry to a number of predetermined settings, and for monitoring apredetermined number of the oversampled bits for each setting; a memoryfor retaining the best setting corresponding to the highest QualityNumber; and a means for updating the said parameters to the bestsetting.

The performance analysis circuitry includes means for receiving a starttrigger to the evaluation run control circuit, and for reporting thebest setting over the control bus.

According to one additional aspect of the invention, there is provided amethod for determining the performance of a cable comprising a boostdevice which receives a differential data signal, deskews and equalizesthe differential data signal according to adjustable parameters, andoutputs a boosted signal, the boost device further comprising aperformance analysis circuitry, including steps of: converting theboosted signal to a single-ended signal; phase-aligning the single-endedsignal with a common clock signal; oversampling the phase-aligned singleended signal and generating a preprocessed data signal; estimating aquality of the preprocessed data signal; and adjusting the adjustableparameters to improve the quality of the preprocessed data signal.

The method as described in claim 29, further comprising an evaluationstep including the steps of: estimating the quality of the preprocesseddata signal and generating a Quality Number indicating said quality;adjusting the adjustable parameters to a number of predeterminedsettings; monitoring the preprocessed data signal for each setting;retaining the best setting corresponding to the highest Quality Number;and updating the adjustable parameters to the best setting.

The method described above further comprises the steps of: starting theevaluation method by receiving a start trigger, and reporting the bestsetting over a control bus.

According to one more aspect of the invention, there is provided a cablefor connecting a transmitting data source device to a receiving datasink device carrying differential signals including: a boost device forboosting at least one of the differential signals, the boost devicecomprising: an input circuit for receiving a raw differential signalfrom the data source device and outputting a recovered signal; a deskewand equalizer circuits with adjustable parameters for processing therecovered signal into a deskewed signal and equalized signal; an outputcircuit for amplifying the deskewed and equalized signal into a boostedsignal and sending the boosted signal to the data sink device; aparameter memory for storing the adjustable parameters; and performanceanalysis circuitry for determining the performance of the cable.

The cable further comprises a control bus, wherein the parameter memoryis accessible from the control bus.

In the embodiments of the invention, the performance analysis circuitryincludes: a differential to single-ended block for converting theboosted signal to a single-ended signal; a linear phase compensator tophase-align the single-ended signal with a common clock signal; anoversampling circuit providing a digital representation of thephase-aligned single ended signal to produce a preprocessed data signal;and a training function circuit for estimating a quality of thepreprocessed data signal, and adjusting the parameters of the deskew andequalizer circuits by changing the adjustable parameters to improve thequality of the preprocessed data signal.

The training function circuit further comprises: a digital circuit forestimating the quality of the preprocessed data signal and generating aQuality Number indicating said quality; an evaluation run controlcircuit for adjusting the parameters of the deskew and equalizercircuitry to a number of predetermined settings, and for monitoring apredetermined number of the oversampled bits for each setting; a memoryfor retaining the best setting corresponding to the highest QualityNumber; and a means for updating the said parameters to the bestsetting.

The performance analysis circuitry includes means for receiving a starttrigger to the evaluation run control circuit, and for reporting thebest setting over the control bus.

A system for calibrating the cable described above is also provided,including: a control computer attached to the control bus of the cable,and a data pattern generator attached to the cable and programmed tosend differential signals into the cable; the control computer isconfigured to send a trigger over the control bus to the performanceanalysis circuitry to start the evaluation run control circuit; toreceive the best settings from the performance analysis circuitry; andto load parameters corresponding to the best settings into the parametermemory over the control bus.

Alternatively, the system for calibrating the cable described abovecomprises: a control computer attached to the control bus of the cable,and a data pattern generator attached to the cable and programmed tosend differential signals into the cable; the control computer isconfigured to send a trigger over the control bus to the performanceanalysis circuitry to start the evaluation run control circuit; and theperformance analysis circuitry is configured to load parameterscorresponding to the best settings into the parameter memory over thecontrol bus.

A corresponding method for calibrating a cable for transmittingdifferential signals is provided, the cable including a boost device fordeskewing and equalizing the differential signals, the boost devicehaving adjustable parameters and a parameter memory, the methodcomprising the steps of: sending a differential data signal into thecable; sending a trigger to the boost device; performing a training runin the boost device, wherein the training run includes the steps ofperforming at least two evaluation runs with different settings of theadjustable parameters and evaluating the results with each of the atleast two settings, retaining the best settings; and storing the bestsettings in the parameter memory.

The step of performing the evaluation run includes the steps of:processing the differential data signal into a deskewed signal;processing the deskewed signal into an equalized signal; and generatinga preprocessed signal, which is a digital representation of theequalized signal.

The step of evaluating includes the steps of: determining the run lengthof contiguous “1” or “0” samples in the digital representation of theequalized signal within a window of at least one bit period; countingthe number of occurrences of selected run lengths during an observationperiod of “N” bits; storing the counted numbers of occurrences incounters according to the selected run lengths; and processing theoutputs of the counters into a Quality Number indicating the quality ofthe equalized signal.

According to yet one more aspect of the invention, there is provided asystem for calibrating a cable for transmitting differential signals,the cable including a boost device for deskewing and equalizing thedifferential signals, the boost device having adjustable parameters anda parameter memory, the system comprising: means for sending adifferential data signal into the cable; means for sending a trigger tothe boost device; means for performing a training run in the boostdevice, including evaluation means for performing at least twoevaluation runs with different settings of the adjustable parameters,evaluating the results with each of the at least two settings andretaining the best settings; and means for storing the best settings inthe parameter memory.

In the system for calibrating the cable described above, the evaluationmeans comprises: means for processing the differential data signal intoa deskewed signal; means for processing the deskewed signal into anequalized signal; and means for generating a preprocessed signal, whichis a digital representation of the equalized signal.

The evaluation means comprises: means for determining the run length ofcontiguous “1” or “0” samples in the digital representation of theequalized signal within a window of at least one bit period; means forcounting the number of occurrences of selected run lengths during anobservation period of “N” bits; means for storing the counted numbers ofoccurrences in counters according to the selected run lengths; and meansfor processing the outputs of the counters into a Quality Numberindicating the quality of the equalized signal.

According to yet one more aspect of the invention, there is provided asystem for calibrating a cable for transmitting differential signals,including a boost device having adjustable parameters and a parametermemory, the system comprising: a network analyzer capable of sending atleast two signals into a cable input and measuring the response at acable output; a computer connected to the network analyzer and to theparameter memory of the cable, the computer having a computer memory;and a computer program code stored in the computer memory for causingthe computer to perform a calibration of the cable by changing theadjustable parameters and storing the results in the parameter memory ofthe cable.

The computer program code causes the computer to perform the calibrationof the cable by performing a training run in the boost device, includingperforming at least two evaluation runs with different settings of theadjustable parameters and evaluating performance of the cable for eachof the settings, and retaining the best settings. The computer programcode also causes the computer to perform said at least two evaluationruns, each evaluation run including: processing the differential datasignal into a deskewed signal; processing the deskewed signal into anequalized signal; and generating a preprocessed signal, which is adigital representation of the equalized signal. The computer programcode further causes the computer to evaluate the performance of thecable for each of the settings by: determining the run length ofcontiguous “1” or “0” samples in the digital representation of theequalized signal within a window of at least one bit period; countingthe number of occurrences of selected run lengths during an observationperiod of “N” bits; storing the counted numbers of occurrences incounters according to the selected run lengths; and processing theoutputs of the counters into a Quality Number indicating the quality ofthe equalized signal. Optionally, the system for calibrating the cablefurther comprises the cable to be calibrated.

A method for operating the system for calibrating the cable describedabove comprises the steps of: (a) measuring differential skew of thedifferential signals at the cable output; (b) changing the adjustableparameters and repeating step (a) when the differential skew is higherthan a predetermined skew threshold; (c) measuring attenuation at eachof a predetermined number of frequencies; (d) changing the adjustableparameters and repeating step (c) when the attenuation is outside apredetermined range at any measured frequency; and (e) storing theparameters in the parameter memory.

The method further comprises the steps of: setting the predeterminedskew threshold to the minimal value observed within a predeterminednumber of repeats of the step (a); setting the predetermined range to avalue close to 0 db, and less than a predetermined limit at each of themeasured frequencies; and setting the predetermined frequency toapproximately a frequency of the differential signals for which thecable is intended.

Thus, an improved programmable cable with embedded power control andboost device is provided along with methods and systems for calibratingthe cable.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings in which:

FIGS. 1A-1C illustrate a high-speed signal to be transmitted through thehigh-speed cable, a distorted bandwidth-limited signal received at theend of the cable (before equalization), and the received signal afterequalization respectively;

FIG. 2A shows timing diagrams of the single ended signal components andthe corresponding differential signal of the differential data on adifferential signaling channel respectively as they might be transmittedby a transmitter into a cable;

FIG. 2B shows example timing diagrams of the single ended signalcomponents and the corresponding differential signal of the differentialdata as they might be received from the end of the cable;

FIG. 3 shows a prior art HDMI (High-Definition Multi-Media Interface)system;

FIG. 4 shows an the HDMI system 10 including an improved HDMI cable 20according to an embodiment of the present invention;

FIG. 5 is a block diagram illustrating the improved HDMI cable 20 ofFIG. 4, including channel boost circuits 100;

FIG. 6 is a more detailed block diagram of the channel boost circuit 100of FIG. 5, including a Differential Deskew Circuit 110;

FIG. 7 shows a simplified block diagram of the Differential DeskewCircuit 110 of FIG. 6, including an adjustable delay block 300;

FIG. 8 shows the preferred embodiment of the adjustable delay block 300of FIG. 7;

FIG. 9 shows a simple RC delay circuit that may be used to introduce thedelay (Td) of FIG. 2B;

FIG. 10 shows simulation results of the RC circuit of FIG. 9;

FIG. 11 shows simulation results of the RC circuit of FIG. 9 with areduced time constant;

FIG. 12 shows a delay circuit made from a cascade of three RC stages;

FIG. 13 shows the waveforms of the trapezoidal input pulse (Vin) and thewaveforms of the delayed pulses after each stage of the circuit of FIG.12;

FIG. 14 shows the same cascaded delay circuit as in FIG. 12, with twobuffers (amplifiers) added;

FIG. 15 shows simulation results of the circuit arrangement of FIG. 14;

FIG. 16 shows a simple follower circuit;

FIG. 17 shows an AC-coupled follower circuit, derived from the simplefollower circuit of FIG. 16;

FIG. 18 illustrates a simplified block diagram of a buffered delay stage400 which may be an embodiment of the delay unit 306 of the adjustabledelay 300 of FIG. 6;

FIG. 19 shows the preferred embodiment of the buffer 404 of the buffereddelay stage 400 of FIG. 18;

FIG. 20 shows a simple N-channel follower;

FIG. 21 shows an alternative embodiment 404B of the delay stage 306;

FIG. 22 shows a simplified transfer function of a cable;

FIG. 23 shows a simplified transfer function of a cascade of anequalizer and a cable;

FIG. 24 shows a system diagram of a representative channel 500,including an optional voltage booster 514, and a power converter 520;

FIG. 25 is a simplified copy 550 of the representative channel 500 ofFIG. 24;

FIG. 26 shows a block diagram of the optional voltage booster 514 ofFIG. 24;

FIG. 27 shows a block diagram of the power converter 520 of FIG. 24;

FIG. 28 illustrates the improved HDMI cable 20 of FIG. 4, showingexternal connections that are available for use in calibrating thecable;

FIG. 29 shows a Real Time Configuration 540, including an expanded boostdevice 544 used in the Real Time Cable Calibration method;

FIG. 30 shows a simplified block diagram of the expanded boost device544 of FIG. 29, including a Linear Phase Compensator 554, anOversampling and Reclocking block 556, and a Training Function 558;

FIG. 31 shows a block diagram of an exemplary implementation of theLinear Phase Compensator 554 of FIG. 30, including a ProgrammableAnalogue Delay 568;

FIG. 32 illustrates data phase shifting in the Programmable AnalogueDelay 568 of FIG. 31, and oversampling in the Oversampling andReclocking block 556 of FIG. 30;

FIG. 33 shows a simplified block diagram of the preferred embodiment 700of the Training Function 558 of FIG. 30;

FIG. 34 shows a high level flow chart of a training run method 800,depicting the operation of the Training Function 558 of FIG. 30;

FIG. 35 shows a flow chart of an exemplary evaluation run method 900further detailing the step 806 of the training run method 800 of FIG.34;

FIG. 36 shows a generic test set up 1000 for Frequency Domain and TimeDomain Calibration methods;

FIG. 37 shows a simplified high level flow chart of an calibrationmethod 1100 that may be used with the generic test set up 1000 of FIG.36 in calibrating the Boost Device 30 in the improved HDMI cable 20 ofFIG. 4;

FIG. 38 shows an alternative embodiment of the invention, in the form ofa modified improved HDMI cable 1200; and

FIG. 39 shows a modified boost circuit 100A of the modified boost device1206.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

FIG. 3 shows a prior art HDMI (High-Definition Multi-Media Interface)system, including a HDMI transmitter Tx (HDMI Source Device), a HDMIreceiver Rx (HDMI Sink Device), and an HDMI cable connecting the Tx andthe Rx.

FIG. 4 shows an HDMI system 10 including an improved HDMI cable 20according to an embodiment of the present invention.

The HDMI system 10 includes the HDMI transmitter Tx (HDMI SourceDevice), the HDMI receiver Rx (HDMI Sink Device), and the improved HDMIcable 20 of the embodiment of the present invention, connecting the Txand Rx.

The improved HDMI cable 20 comprises an embedded boost device 30,details of which are described in the following, and a basic (passive)HDMI cable 40. The boost device 30 is located near the end of theimproved HDMI cable 20 closest to the HDMI receiver Rx. Without limitingthe generality of the application, the improved HDMI cable 20 may beused to connect a DVD player (an example of an HDMI Source Device) to aTelevision Screen (an example of an HDMI Sink Device).

FIG. 5 is a block diagram illustrating the improved HDMI cable 20 thatextends between the HDMI transmitter Tx and the HDMI receiver Rx,including the boost device 30 of FIG. 4. Also shown are HDMI inputs 50extending from the Tx to the boost device 30 through the basic HDMIcable 40, HDMI outputs 52 extending from the boost device 30 to the Rx,and a group of Other HDMI Signals 54 extending directly from the Tx tothe Rx through the basic HDMI cable 40. The basic HDMI cable 40 includesthe HDMI inputs 50 and the Other HDMI Signals 54.

The HDMI inputs 50 provide the connections that couple HDMI signals fromthe HDMI transmitter Tx (FIG. 4) over the wires of the basic HDMI cable40 to inputs of the boost device 30. The HDMI inputs 50 include four (4)signal pairs:

-   a Transition Minimized Differential Signaling (TMDS) Channel Input    0;-   a TMDS Channel Input 1;-   a TMDS Channel Input 2; and-   a Clock Channel Input.

Similarly, the HDMI outputs 52 include four (4) signal pairs of boostedHDMI signals:

-   a TMDS Channel Output 0;-   a TMDS Channel Output 1;-   a TMDS Channel Output 2; and-   a Clock Channel Output.

The HDMI outputs 52 couple the boosted HDMI signals from the boostdevice 30 over a short connection to the HDMI receiver Rx.

A Programming input 56 and a +5V Power signal 58 is coupled from theOther HDMI Signals 54 to the boost device 30. Not shown in the figureare physical features such as device carrier(s) and connectors which maybe part of the improved HDMI cable 20.

The boost device 30 includes a number of channel boost circuits 100, aparameter memory 102. In the preferred embodiment of the invention, theboost device includes four (4) channel boost circuits 100 as shown inFIG. 5, each to boost the signal of one of the TMDS Channel 0, the TMDSChannel 1, and the TMDS Channel 2.

Each channel boost circuit 100 includes an HDMI Input Circuit 106 and anHDMI Output Circuit 108. Each channel boost circuit 100 advantageouslyfurther includes a Differential (intra-pair) Deskew Circuit 110 foradjusting an existing time skew of the two polarities of a differentialdata signal propagating through the basic HDMI cable 40 and an EqualizerCircuit 112 to compensate for the limited bandwidth characteristics ofthe basic HDMI cable 40. Each channel boost circuit thus provides atransfer function from the respective HDMI Input to the correspondingHDMI Output with characteristics designed to compensate for thedegradation of the corresponding differential pair in basic cable 40.

The boost device 30 may be powered by the +5V Power signal 58, and bypower derived from the HDMI Outputs 52 as will be described in detailbelow. The power for the operation of the boost device 30 is entirelyderived from signals carried in the improved HDMI cable 20, and suppliedby the HDMI transmitter Tx and/or the HDMI receiver Rx.

In a cable carrying differential signals, i.e. where each signal iscarried over a pair of wires, manufacturing tolerances commonly resultin slight differences between the lengths of the wires and connectorsused for each channel. The result will be a different delay through thecable for each of the pair. Such differential (intra-pair) skew degradesthe received signal (see FIGS. 2A and 2B above). Elimination ofintra-pair skew may be accomplished by adding delay to the signalpassing through the shorter of the pair of wires by the appropriateamount so that it is aligned with the signal passing through the longerof the pair. In accordance with the embodiments of present invention,intra-pair skew is eliminated with the help of the Differential DeskewCircuit 110, which is digitally programmable as will be described in thenext sections. The parameter memory 102 is used to retain the deskewsettings of the Differential Deskew Circuit 110, once they aredetermined in a programming (calibration) setup method.

Similarly, cables present different bandwidth characteristics, whichdepend on the length and the physical construction of the cable. Thelimited bandwidth may be compensated (to some extent) by the EqualizerCircuit 112 which is also digitally programmable. The equalizer settingsmay similarly be retained in the parameter memory 102. The propersettings for both the Differential Deskew Circuit 110 and the EqualizerCircuit 112 may be determined in a programming (calibration) setup atthe time of production, and loaded into the parameter memory 102 throughthe Programming Input 56. The programming setup method will be describedin more detail further below (FIGS. 29 to 37).

FIG. 6 shows a more detailed block diagram of a single instance of thechannel boost circuit 100 of FIG. 5, comprising the HDMI Input circuit106, the Differential Deskew circuit 110, the Equalization circuit 112,and the HDMI Output circuit 108.

The input to the HDMI Input circuit 106 is a raw input signal (pair) 116(one of the HDMI Inputs 50, FIG. 5). The HDMI Input circuit 106 outputsa “recovered signal” (pair) 118 that is input to the Differential Deskewcircuit 110. The Differential Deskew circuit 110 outputs a “deskewedsignal” (pair) 120 that is input to the Equalization circuit 112. TheEqualization circuit 112 outputs an “equalized signal” pair 122 that isinput to the HDMI Output circuit 108. And finally, the HDMI Outputcircuit 108 outputs a “boosted signal” (pair) 124 that is one of theHDMI Outputs 52 (FIG. 5).

Also shown in FIG. 6 is the Parameter Memory 102, which is shared amongall channel boost circuits 100 of the boost device 30. It is connectedto a deskew parameter input 126 of the Differential Deskew circuit 110,and separately to an equalization parameter input 128 of theEqualization circuit 112.

Differential Deskewing Circuit 110

As indicated above, the intra-pair differential skew delay may becompensated by inserting a delay element having a delay of Td in thepath of V+ (in the case of the example of FIG. 2B), or in the path of V−in the opposite case (if the input V+ signal was delayed with respect toV−), or neither if there was no skew present.

FIG. 7 shows a simplified block diagram of the Differential DeskewCircuit 110 of FIG. 6 in which the differential skew is removed(compensated). The same reference numerals are used to indicate thedifferential inputs and outputs (the recovered signal 118 and thedeskewed signal 120 respectively, each with a positive [V+] and anegative [V−] terminal), and the control input for the deskew parameters(126).

As shown in FIG. 7, the Differential Deskew circuit 110 includes anadjustable delay 300 with a (single-ended) input 302 and an output 304,and six ON/OFF switches S1 to S6. The adjustable delay 300 includes anumber of delay stages 306. The switch S1 is connected between thepositive terminal of the differential input (the recovered signal 118V+) and the positive terminal of the differential output (the deskewedsignal 120 V+). Similarly, the switch S6 is connected between thenegative terminal of the differential input (the recovered signal 118V−) and the negative terminal of the differential output (the deskewedsignal 120 V−). The switches S2 and S4 are connected between the input302 of the adjustable delay 300 and the positive (V+) and negative (V−)terminals respectively of the recovered signal 118. Similarly, theswitches S3 and S5 are connected between the output 304 of theadjustable delay 300 and the positive (V+) and negative (V−) terminalsrespectively of the deskewed signal 120.

The scheme allows the single adjustable delay 300 to correct for bothpositive and negative differential skew. In effect, the singleadjustable delay 300 is sufficient to compensate positive or negativedifferential skew (where either the positive signal or the negativesignal is delayed with respect to the other), by switching it (theadjustable delay 300) into either the negative or the positive signalpath respectively. For example, to pass the positive signal V+ throughthe adjustable delay 300 (which is made of a cascade of delay units, tobe described in detail below) the switch states are as follows: S1=OFF,S2=ON, S3=ON, S4=OFF, S5=OFF, and S6=ON. To pass V− through theadjustable delay 300 the switch states are as follows: S1=ON, S2=OFF,S3=OFF, S4=ON, S5=ON, S6=OFF. To switch the adjustable delay 300 out ofboth the V− and the V+ paths, thus providing no adjustment of thedifferential delay, the switch states are as follows: S1=ON, S2=OFF,S3=OFF, S4=OFF, S5=OFF, S6=ON.

The solution of the deskew problem presents two challenges. The first isto make a suitable delay, the second is to tune the delay. Making thedelay is a challenge because the unit should have a wide enoughbandwidth to pass the signals but at the same time the delay block hasto present a useful delay. The wide bandwidth of a single delay stagenaturally results in little delay, so a cascade of stages is required toachieve a sufficient delay.

A cascade of digital delay stages, including digital switches and adecoder to provide binary addressable selection of the overall delay,are described in U.S. Pat. No. 6,268,753. However, the present inventionrequires an adjustable delay circuit to delay a high-speed analogsignal.

Issues to be solved with a cascade of analog delay stages in theproposed configuration of FIG. 7 for differential skew compensation,include the need to provide unity gain, as well as preserve the highbandwidth required.

Among the prior art, several digital delay compensation schemes aredisclosed, but only few circuits providing adjustable delay for analogsignals. For example, the use of a follower circuit in parallel with again stage to boost the high frequency response of a digital circuit istaught in U.S. Pat. No. 5,739,713. U.S. Pat. No. 6,525,568 teaches aphase shifting (delay) stage that includes an RC (resistor-capacitor)element followed by parallel gain stages of nominally −1 and +2 gain,their outputs added together to provide overall unity gain with aparticular complex frequency transfer function. In the United StatesPatent Application 20050083130 a high performance amplifier is proposedwhich includes a delay element to compensate for signal propagationdelay that may exist in alternative signal paths.

FIG. 8 shows the preferred embodiment of the adjustable delay block 300of FIG. 7 as a cascade of eight analog delay stages (“Delay Units”) 306in combination with an analog selector stage 308 as a solution toimplement the adjustable delay 300. The eight delay units 306 areconnected in series (cascaded), the output of each delay unit 306 beinginput to the analog selector stage 308. The first delay unit 306 of thecascade provides the input of the adjustable delay 300 (IN 302).

The deskew parameters control signal (deskew parameter input 126)includes a 3-bit binary signal connected to the analog selector stage308 for selecting one of its inputs to be switched through to the outputof the adjustable delay 300 (OUT 304).

An exemplary complete circuit of the single delay unit 306, which may becascaded to provide a unit of delay each, for the adjustable delay 300is shown in FIG. 18 below.

To help in understanding the circuitry of the single delay unit 306, astep-by-step description of the issues to be solved, and possiblesolutions, is presented first.

FIG. 9 shows a simple RC delay circuit that may be used to introduce thedelay (Td) of FIG. 2B. The circuit of FIG. 9 is a single ended circuitcomprising a resistor R1, a capacitor C1, and input and output terminals(signals Vin and Vout), as well as a ground (0). The capacitor C1 isconnected between Vout and ground, and the resistor is connected betweenVin and Vout. Making a circuit with an RC delay as shown in FIG. 9 willsucceed in delaying the signal but it will also filter the signal.

The impact of the RC circuit of FIG. 9 on a pulse is seen fromsimulation results shown in FIG. 10. FIG. 10 shows two simulated waveforms, a trapezoid input pulse Vin, and an output pulse (Vout), thatresults from passing the trapezoid input pulse through the simple RCdelay circuit of FIG. 9. The trapezoid input pulse (the signal Vin) isdelayed and filtered (distorted) into the output signal Vout. The delayand the filtering action are clearly seen. While the delay is desirable,the filtering action causes dispersion and distortion of the pulse.

To reduce the filtering action of the circuit the RC time constant maybe reduced. The simulated result is shown in FIG. 11. The simulationshown in FIG. 11 is analogous to the simulation shown in FIG. 10, butwith a reduced time constant in the simulated delay circuit. Reducingthe time constant helps to increase or maintain the bandwidth (note theslopes of both the input and output pulses) but as shown in FIG. 11 thesignal delay introduced is lower.

In the simulation of FIG. 11 the pulse width is 0.7 second and the RCtime constant in the circuit is 79 ms. The long pulse duration and thelong RC time constant were chosen merely for convenience in thesimulations to study the effects of circuit choices, and are notrepresentative of the time scales of the embodiment.

One method of attempting to regain the delay (as shown in FIG. 10 withrespect to the circuit of FIG. 9 with the original time constant) is tocascade a number of RC stages. This is shown in FIG. 12. Shown in FIG.12 is a delay circuit made from a cascade of three RC stages, comprisingthe components R2, C2, R3, C3, R4, and C4, each RC stage havingindividually the same time constant of 79 ms. The signals after thefirst and second stages are labeled V1 and V2 respectively. The inputand output of the circuit as a whole are labeled Vin and Vout.

The result of simulating the circuit of FIG. 12 with a trapezoidal inputpulse is shown in FIG. 13 which shows the waveforms of the trapezoidalinput pulse (Vin) and the waveforms of the delayed pulses after eachstage of the circuit of FIG. 10 (V1, V2, and Vout). The resulting finalwaveform Vout is delayed but it is considerable reduced in amplitude anddispersed.

In order to remove the loading effect of subsequent stages, each stagemay be buffered as shown in FIG. 14. Shown in FIG. 14 is the samecascaded delay circuit as in FIG. 12, but two buffers (amplifiers) areinserted, a buffer “Buf1” between R1 and R2, and a buffer “Buf2” betweenR2 and R3. As a result, the intermediate signals V1 and V2 are notattenuated by the loads of the subsequent stages.

The simulation results for the circuit of FIG. 14 are shown in FIG. 15.They show that the circuit arrangement of FIG. 14 achieves the desiredgoal of introducing significant delay while the distortion in the pulseis kept to a minimum. In this simulation, the 0.7 second trapezoidalinput pulse is delayed by approximately 77 ms per stage.

In a mathematical sense, the pulse has been transformed by a cascade ofsingle pole unity gain stages, the transfer function of each stagebeing;

H(s)=1/(1+s/p) where p[Rads]=1/(RC)

or p[HZ] is 1/(2πRC)

The goal of the circuitry is to delay the pulse by up to about half thepulse width (bit width). In the case illustrated in the simulation ofFIG. 15, the required delay would be approximately 0.35 seconds. Toachieve this delay with the scheme shown in FIG. 14, this would requireapproximately five stages. In the simulation shown in FIG. 15 the RCtime constant is set to 79 ms which sets the pole frequency at 1/(2π79ms)=2 Hz. Thus, with a pulse width of 0.7 seconds (the input pulse Vin)a stage with a pole frequency of 2 Hz will produce suitable delays withacceptable filtering on the pulse. A simple approximation to calculatethe position of the pole for a system with a bit rate of N bits persecond (Nbps) is to set the pole in each stage at 3*N Hz. For example,with a data rate of 1 Gbps, a stage with a pole at approximately 3 GHzwould be needed.

Having shown how an appropriate delay per stage may be achieved usingsimple RC stages, it remains to be shown how a suitable buffer amplifier(Buf1, Buf2 in FIG. 12) may be constructed. To make a unity gain bufferwith unity gain up to 3 GHZ is a challenge even on an advanced CMOSprocesses. A starting point would be to use a simple follower circuit asshown in FIG. 16. The simple follower circuit of FIG. 16 includes anN-channel MOS field-effect transistor (MOSFET) M1 connected in serieswith a current source I1. The drain of the transistor M1 is connected toground (0), while the positive terminal of the current source I1connects to the supply voltage VDD. The circuit input (IN) is connectedto the gate of the transistor M1, and its source provides the circuitoutput (OUT).

In this well-known circuit the output OUT follows the input IN with again of approximately one. The first limitation with this circuit isthat the output is typically level shifted by 0.6 volts or so. Thislevel shifting is a problem if a number of stages are to be cascadedbecause the successive level shifts will cause the output to rise to thesupply voltage and thus the signal is clipped. To solve this limitation,AC-coupling is added to the simple follower as shown in FIG. 17. Thecircuit shown in FIG. 17 is an AC-coupled follower circuit, derived fromthe simple follower circuit of FIG. 16 by the addition of a capacitor C5between the circuit input (IN) and the source of the transistor M1, anda resistor R5 between the source of the transistor M1 and a bias supply“BIAS” that provides a positive bias voltage.

With AC-coupling, the fact that the output of the stage is level shiftedup from the bias level set by “BIAS” in FIG. 17 becomes unimportant whenstages are cascaded, because this level shift is stored as a constantdrop across the input capacitor of the next stage. This essentiallyresets the average input voltage at each stage to be the bias voltageset by the bias supply (BIAS) shown in FIG. 17.

A further limitation of this circuit comes from the non-zero outputconductance of the transistor M1. The gain of the follower is given bygm1/(gm1+gds1). Here gm1 is the small signal transconductance and gds1is the small signal output conductance of M1. Clearly, for all values ofgds1 greater than zero the gain of the stage is less than one. When fastwideband circuits are required, the length of the MOSFET M1 is reducedto close to minimum. This causes gds1 to increase to a point where thegain is now tending to 0.9 or so. A cascade of these stages woulddramatically reduce the magnitude of the incoming signal.

One possible architecture which corrects for this reduced stage gain isshown in FIG. 18 which illustrates a simplified block diagram of abuffered delay stage 400, which may be an embodiment of the delay unit306 of the adjustable delay 300 (FIG. 6).

The buffered delay stage 400 comprises a unit gain amplifier (buffer)404. The buffer 404, having an input 410 and an output 412.

The buffer 404 includes two amplifiers in parallel, a follower stage414, having a gain of approximately 0.9 and a supplementary stage 416with a gain of approximately 0.1, both amplifiers having the samefrequency response (expressed mathematically by the pole 1/(1+s/p). Bothamplifiers (414 and 416) share the input 410 of the buffer 404, andtheir outputs are summed into the output 412.

The buffered delay stage 400 provides an inherent delay (implicit in thepoles p of the transfer functions), and by virtue of the amplifiers,provides the isolation from the next delay element in the cascade, asdescribed earlier (FIG. 14). Note that in very high-speed operation, noexplicit RC delay element is needed if the (by necessity limited)frequency response of the buffer 404 is designed to provide the requireddelay.

The buffered delay stage 400 receives the input signal VIN of thebuffered delay stage 400 connected to the input 410 of the buffer 404;and the output 412 of the buffer 404 generates the output signal VOUT ofthe buffered delay stage 400.

The preferred embodiment of the buffer 404 including its componentamplifiers (the follower stage 414 and the supplementary stage 416), isshown in detail in FIG. 19, as a circuit based on an N-well CMOSprocess.

The follower stage 414 is an AC-coupled circuit, similar to theAC-coupled follower circuit of FIG. 17. It comprises a P-channelfollower transistor M2, a biasing resistor R6, a coupling capacitor C6,and a bias supply “BIAS1”.

The supplementary stage 416 includes a N-channel amplifying transistorM3, and two P-channel transistors M4 (functioning as a diode) and M5(functioning as a current source); a biasing resistor R7; a couplingcapacitor C7; a shunt capacitor C8; and a bias supply “BIAS2”.

The bias voltages of “BIAS1” and “BIAS2” are adapted to the circuitfunctions and the technology as required.

The input 410 of the buffer 404 is connected through the couplingcapacitor C6 to the gate of the transistor M2, and through the couplingcapacitor C7 to the gate of the transistor M3. The positive terminal ofthe bias supply “BIAS1” is fed to the gate of the transistor M2 throughthe biasing resistor R6. Similarly, the positive terminal of the biassupply “BIAS2” is fed to the gate of the transistor M3 through thebiasing resistor R7. The negative terminals of “BIAS1” and “BIAS2”, thedrain of the transistor M2, the source of the transistor M3, and oneterminal of the shunt capacitor C8 are connected to ground. The otherterminal of the shunt capacitor C8 is connected to the gate of thetransistor M3. The source of the follower transistor M2 is connected tothe drain of the current source transistor M5 and the output 412 of thebuffer 404. The drains of the transistors M3 and M4 are connectedtogether, and also to the gate of the transistor M4. The sources of thetransistors M4 and M5 are connected to the supply voltage VDD.

Functionally, the signal of the input 410 of the buffer 404 is amplifiedby the follower stage 414 with a gain of about 0.9, the transistor M5(in the supplementary stage 416) providing a current source load to thefollower transistor M2. The function of the supplementary stage 416 isto amplify a portion of the same input signal (the portion being definedby the ratio of the coupling capacitor C7 to the shunt capacitor C8) inthe transistor M3 into a varying current that is mirrored through thetransistors M4 and M5, and so providing a varying current source load tothe follower transistor M2. Thus, both the follower stage 414 and thesupplementary stage 416 contribute to the signal at the output 412 ofthe buffer 404, their individual contributions effectively being addedas indicated in FIG. 18 above.

The gain of the P-channel follower circuit (414) is essentially unityexcept for the output conductance (gds) of the P-channel device (M2).Because of the requirement for speed a short P-channel device isrequired and thus the device has a large output conductance and the gainfalls toward 0.9. With a cascade of 5 stages the signal would havefallen to 60% of its original value. To boost the gain of the simplefollower at channel data rates, the parallel signal path is provided inthe form of the supplementary stage 416.

As described above, the buffer 404 includes a second path (thesupplementary stage 416) for the input signal (410) to arrive at theoutput 412. This extra path is through C7, M3, M4, and M5. In this path,the high frequency input signal is passed though C7 and a fraction ofthe signal is presented at the gate of M3. This fraction is changed bychanging the size of the shunt capacitor C8. The current in M3 is set toa nominal value with a bias circuit (“BIAS2”). When the input signalarrives at the gate of M3 it varies the current in M3. This currentvariation is sourced by the diode connected device (M4) which thenmirrors the current change to M5. Finally M5 changes the current in M2so the end result is that changing the input signal changes the currentin M2. Changing the current level in M2 changes the overdrive in thedevice and thus changes the output voltage. In summary a positive changeat the input 410 causes a positive change at the output 412 due tocurrent steering in the parallel path. At the same time there is apositive change at the output due to the simple follower action throughM2. The overall change in the output is calculated by summing thecontributions from the Parallel (supplementary stage 416) and Main(follower stage 414) paths. If the main path is producing a gain of 0.9the parallel path may be tuned to provide a gain of 0.1 by changing thevalue of C8. Once adjusted to unity, the gain of the stage remainsstable over Process, Supply Voltage, and Temperature to within about twopercent of its nominal value.

The buffer circuit 404 of FIG. 19 meets the following requirements.

An overall gain of unity and thus cascading does not amplify or reducethe signal;

Capable of very wideband operation (pole at 2GHz to 10GHz) for minimaldistortion; and

Input and Output levels of a cascade of stages stay within a suitablerange.

Some typical values for the implementation of the buffer 404 are: R6=200k, R7=200 k, C6=200 f, C7=200 f, C8 is tuned to adjust the overall gainof the circuit to unity.

An embodiment of the buffer, equivalent to the buffer circuit 404 shownin FIG. 19, may be created by starting with a simple N-channel follower(instead of the P-channel follower of FIG. 16, that has led to thecomplete buffer implementation shown here in FIG. 18). The simpleN-channel follower is shown in FIG. 20.

For a CMOS process with a P-Well technology the configuration shown inFIG. 19 would be the preferred implementation in that the bulk of theN-channel Mosfet would be free to be tied to the source as shown in FIG.20. For the more standard CMOS processes with N-Well technology thecircuit of the buffer of FIG. 19 would cause additional challengesbecause the bulk connection on the N-channel MOSFET is tied to ground.This grounded bulk causes a varying source-to-bulk potential in thetransistor and further degrades the gain of the stage from 0.9 and thisreduced gain makes it more difficult to maintain the overall gain of thestage at unity.

An alternative configuration for making the buffer stage for the delaystage 306 is shown in FIG. 21, which shows a modified buffer 404B. Themodified buffer 404B is similar to the buffer 404 and also uses twoparallel paths for the input (410) to output (412) signal, i.e. thefollower stage 414 and a modified supplementary stage 416B. The modifiedsupplementary stage 416B performs the same function as the supplementarystage 416, but is implemented somewhat differently.

The supplementary stage 416B comprises five N-channel transistors (M6,M7, M8, M9, and M10) and two P-channel transistors (M11, M12), a biasingresistor R8, a coupling capacitor C9, a shunt capacitor C10, and acurrent sink 12.

The components of the supplementary stage 416B are variously connectedto each other, ground, and VDD as listed in the following:

-   the sources of the N-channel transistors (M6 to M10) and one lead of    the shunt capacitor C10 are connected to VDD;-   the sources of the P-channel transistors (M11 and M12) as well as    the negative terminal of the current sink 12 are connected to    ground;-   the transistors M6, M9, and M11 are each connected in diode mode,    i.e. their gates are shorted to their drains;-   the drain/gate of the transistor M6 is connected to the positive    terminal of the current source I2, the gate of the transistor M7,    and through the biasing resistor R8 to the gate of the transistor    M8;-   the drain of the transistor M7 is connected to the drain/gate of the    transistor M11 and to the gate of the transistor M12;-   the gate of the transistor M8 is further connected to the shunt    capacitor C10, and through the coupling capacitor C9 to the input    signal (410);-   the drain of the transistor M8 is connected to the drain/gate of the    transistor M9, to the gate of the transistor M10, and the drain of    the transistor M12; and lastly-   the drain of the transistor M10 is connected to the drain of the    transistor M2 of the follower stage 414 as well as the output 412.

In this configuration (the supplementary stage 416B), the circuitryformed by the current sink 12 and the transistor M6 provides a biasvoltage from which, through the resistor R8 the operating point of thetransistor M8 is set; and further, through the current mirror formed byM11 and M12, the current drawn by the transistors M8 and M9 is set.

The input signal (410) fed through the coupling capacitor C9 to the gateof the transistor M8 modifies the current in the transistor M8 and thusmodifies the current in the transistor M9 (the current in M9 is thedifference between the constant current set in M12 and the signaldependent current in M8), and consequently the current in the transistorM10 due to the mirroring of M9 and M10. The variation of current in thetransistor M10, which is in series with the transistor M2 in thefollower stage 414, has the same effect as that described earlier forthe variation in the equivalent transistor M5 of the originalsupplementary stage 416.

Again, a configuration similar to the circuitry of the buffer 404B maybe produced if one starts with an N-channel follower as shown in FIG. 20above.

Equalization Circuit 112

The output of the cable shows a low pass filtered response and thusthere is significant distortion to the incoming signal. The challengingfeatures of the distorted signal are reduced rise times and the factthat a single data bit change does not cause the signal to traverse thesignal range.

Typical waveforms at the input and the output of a cable have been shownin FIG. 1A and 1B above, for illustration of this common problem. Thelimited bandwidth of the cable suppresses the high frequency componentsof the data signal. A simplified transfer function of a cable isillustrated in FIG. 22 to show the reduction in gain at highfrequencies.

The high frequency suppression is conventionally solved by placing anequalizer in the cable (or in the receiver). The equalizer provides anincreased gain at the higher frequencies so the cascading of thetransfer functions results in a flat unity gain transfer function overthe frequencies of interest as shown in FIG. 23.

Such an existing approach to solving the problem is described, e.g. in aU.S. Pat. No. 6,819,166. This existing implementation describes anequalizer with a variable transfer function, and a method of detectingthe level of high frequency suppression in the cable such that theequalizer can be tuned to accurately offset the impact of this.

In the embodiment of the present invention, a tunable equalizer isprovided in the Equalization circuit 112 (FIG. 6). Instead of providinginfinitely variable equalization, a finite number of discrete settingsare implemented, which may be selected under control of the equalizationparameter input 128.

FIG. 24 shows a system diagram of a representative channel 500 thatincludes a typical differential driver circuit 502 as may be found inthe HDMI Source Device (Tx) of FIG. 4; a typical differentialtermination circuit 504 as may be found in the HDMI Sink Device (Rx) ofFIG. 4; and a boost circuit 506. The boost circuit 506 is a moredetailed depiction of an implementation of the boost circuit 100 of FIG.6 according to the preferred embodiment of the invention.

The typical differential driver circuit 502 is conventional andcomprises a differential pair of N-channel MOSFETs(metal-oxide-semiconductor field-effect transistor) M13 and M14 and acurrent source I3. The sources of the transistors M13 and M14 are tiedtogether and connected to the common ground through the current source13 which is adjusted to supply a current of approximately 10 mA inaccordance with the HDMI specification. The gates of the transistors M13and M14 are driven with a differential signal (not shown) which may beone of the TMDS data signals if the channel 500 is a TMDS data channel,or the clock signal if the channel 500 is the clock channel. The outputof the typical differential driver circuit 502 is the raw input signal(pair) 116 of the boost circuit 100 of FIG. 6, embodied in the boostcircuit implementation 506.

The typical differential termination circuit 504 comprises two resistors(R9 and R10, typically each having a value of 50 Ohm) which are tied toa supply voltage (typically 3.3V) that is internal to the HDMI sinkdevice. The input of the differential termination circuit 504 (signalends of the resistors R9 and R10) is the “boosted signal” (pair) 124which is also the output of the boost circuit 100 of FIG. 6, embodied inthe boost circuit implementation 506.

Not shown in FIG. 24 is the basic (passive) HDMI cable 40 that carriesthe raw input signal (pair) 116 from the typical differential drivercircuit 502 to the boost circuit 100 (506).

By way of explaining the operation of the representative channel 500,let us first consider the case without the boost circuit 506,corresponding to the previously shown prior art diagram of FIG. 3.

In this prior art case, the output of the typical differential drivercircuit 502 (the raw input signal 116) would be connected to the input(124) of the typical differential termination circuit 504, directlythrough the basic HDMI cable. A current, its magnitude determined by thecurrent source 13 (10 mA), flows from the supply voltage 3.3V throughone or the other of the resistors R9 and R10; over the corresponding oneor the other conductor of the differential pair (116 and 124); throughone or the other of the transistors M13 and M14 (of which one isswitched on while the other is switched off by the differential signal);and through the current source 13 to ground. Which of the one or otherof resistors, conductors, and transistors, is determined by the state ofthe differential signal. A logical “0” signal may cause substantiallyall of the current to flow through the transistor M13 and the resistorR9 while a logical “1” would cause the current to flow through M14 andR10. As a result, the voltages at the signal ends of the terminationresistors may vary between 3.3V and 2.8V, presenting thus a differentialsignal of about ±0.5V. In practice, the differential signal may be lowerdue to loss in the cable and loading at the termination.

It is a function of the boost circuit 506 according to the invention, tomimic the behavior of the typical differential termination circuit 504at the input of the boost circuit 506, and the behavior of the typicaldifferential driver circuit 502 at its output.

The boost circuit 506 shown in FIG. 24 includes an HDMI input circuit508 (showing a detailed implementation of the HDMI input circuit 106 ofFIG. 6), an HDMI output circuit 510 (showing a detailed implementationof the HDMI output circuit 108 of FIG. 6), and a processing block 512that includes the Differential Deskew circuit 110 and the Equalizationcircuit 112 of FIG. 6.

The boost circuit 506 may further include an optional Voltage Booster514 with an input 516 and an output 518. When the optional VoltageBooster 514 is not provided, it is simply bypassed, that is the input516 is directly connected to the output 518.

The HDMI input circuit 508 is very similar to the typical differentialtermination circuit 504, including two 50 Ohm resistors R11 and R12,tied to a supply voltage V3, and having signal ends that are connectedto the raw input signal 116. The differential voltage signal thatdevelops by virtue of a switched current alternating through theresistors R11 and R12 is simply connected as the “recovered signal” 118to the input of the Deskew Circuit 110 in the processing block 512 (seealso FIG. 6). The supply voltage V3 is supplied by a filter capacitorC11 that is connected to the output 518 of the optional Voltage Booster514.

The processing block 512 receives the “recovered signal” 118 from theHDMI input circuit 508 and, after processing the signal in theDifferential Deskew circuit 110 and the Equalization circuit 112,outputs the “equalized signal” 122. Power is provided to the processingblock from the +5V supply. The processing block 512 also includes aPower Converter 520 which may be used to efficiently convert thesupplied power of +5V to a lower voltage that is then supplied to theDifferential Deskew circuit 110 and the Equalization circuit 112.

The HDMI output circuit 510 has some similarity with the typicaldifferential driver circuit 502. The HDMI output circuit 510 comprisesN-channel MOSFETs M15 and M16 (or alternatively, P-channel MOSFET M15and M16) which are analogous to the transistors M13 and M14 of thetypical differential driver circuit 502. The sources of the transistorsM15 and M16 are tied together (thus forming a transistor pair) andconnected to the drain of an N-channel MOSFET M17. The gates of thetransistor pair M15 and M16 are connected to the “equalized signal” pair122. The drains of the transistor pair M15 and M16 are connected to, anddrive, the differential “boosted signal” (pair) 124 that is connected tothe typical differential termination circuit 504 in the HDMI sink (Rx).

The HDMI output circuit 510 further includes an N-channel MOSFET M18 anda current source 14. The transistor M17, whose drain is connected to thesources of the transistor pair M15 and M16, has its source connected toa voltage node V4. The gate of the transistor M17 is connected to a nodeV5 that connects the gate and the drain of the transistor M18 and thenegative terminal of the current source 14 whose positive terminal isconnected to the +5V supply. The source of the transistor M18 isconnected to a bias voltage node “BIAS4”. In effect, the transistor M18is configured as a diode between the BIAS4 and the negative terminal ofthe current source 14, providing the voltage V5 to the base of thetransistor M17 such that the resulting voltage V4 at the source of thetransistor M17 mirrors the BIAS4 voltage.

The operation of the HDMI input and output circuits 508 and 510 may bedescribed by considering their common-mode behavior first.

FIG. 25 is a simplified copy 550 of the representative channel 500 ofFIG. 24, for the purpose of illustrating the common mode functionalityof the HDMI input and output circuits 508 and 510 through which power isharvested from the signals. Shown in thick lines are two current pathsextending from the 3.3V supply in the typical differential terminationcircuit 504 to the common ground in the typical differential drivercircuit 502. The solid thick line indicates the current path when thetransistors M13 and M15 are turned on, and the transistors M14 and M16are turned off. The dotted thick line shows an alternate current pathwhen the respective transistors are in the opposite state.

Tracing the solid thick line, a current flows from the 3.3V supplythrough the resistor R9; the transistor M15; the transistor M17; theresistor R11; the transistor M13; and the current source 13, to ground.The optional voltage booster 514 is bypassed (not shown in thisillustration), but will be described in a subsequent FIG. 26. Themagnitude of the current in the solid thick line is determined by thecurrent source 13, approximately 10 mA, and will cause voltage drops ofapproximately 0.5V in each of the resistors R11 and R9. The voltagedrops in the transistors M13 and M15 is controlled by the voltagepotential at the intermediate point along the current path, i.e. thevoltage nodes V3 and V4 which are equal in the absence of the optionalvoltage booster 514. The voltage level of V4 is designed to besubstantially the same as the bias voltage BIAS4 which may beconveniently set at about 2.0V, that is sufficiently low to avoidsaturating the transistor M15. The transistor M17, carrying the entirecurrent of 10 mA does almost saturate and its voltage drop is small. Theoperating point of M17 is set by the mirroring transistor M18 whosecurrent (controlled by the current source 14) may be for example 0.1 mA.By choosing the geometries of M18 and M17 to match the ratio of currentsources (0.1 to 10 mA, or 1:100), the voltage drop from gate to sourceof the transistor M17 will be the same small value as that of M18.

The filter capacitor C11 may have a capacitance of 10 nF. Its purpose isto smooth the voltage level of V3 (which is the same as V4 if theoptional voltage booster 514 is not present) when the current pathswitches back and forth between the paths shown in solid and dottedlines. Furthermore, the switching back and forth of the current pathbetween R11 and R12 does not necessarily occur at precisely the sameinstants as the switching between R9 and R10, because of the delaysintroduced by the Processing Block 512 whose output controls theswitching action of the transistors M15 and M16. The resulting currentspikes are also smoothed by the filter capacitor C11.

FIG. 26 shows a block diagram of the optional voltage booster 514 ofFIG. 24.

The input 516 of the voltage booster 514 is connected to the voltagenode V4, and the output 518 is connected to the capacitor C11 and thevoltage node V3 as shown in FIG. 24.

The voltage booster 514 functions as a charge pump and is similar to the“High-Efficiency CMOS Voltage Doubler” by Favrat et al, IEEE J. SolidState Circuits, vol. 33, no. 3, pp. 410-416, March, 1998. The circuitincludes two capacitors C12 and C13, and two “collector” switches S7 andS8, and two “deposit” switches S9 and S10. The capacitor C13 is disposedbetween the voltage node V4 and ground. The capacitor C12 is a “flying”capacitor having a positive (+) terminal connected to the switches S7and S9, and a negative (−) terminal connected to the switches S8 andS10. The switches are periodically closed and opened, driven by a signalfrom a pumping oscillator (not shown) whose frequency may beconveniently chosen to be around 100 MHz. The switches are operated insuch a way that the collector switches S7 and S8 are closed while thedeposit switches S9 and S10 are opened, and vice versa. The switches mayconveniently be implemented in P-channel and/or N-channel MOSFETs. Thesizes of the capacitors are not critical, nor is the ratio of sizes.Satisfactory results have been obtained in simulations of the circuitwith the following values: C11=10 nF; C12=1 nF; and C13=10 nF.

When the collector switches are closed (during a “collector phase”), S7connects the positive terminal of C12 to V4 and the negative terminal toground, thus placing the capacitor C12 in parallel with the capacitorC13.

In the collector phase, the flying capacity or C12 “collects” somecharge from the capacitor C13 at the voltage node V4. Recall that thenode V4 is fed by current from the transistor M17 (FIG. 24) whichcontinuously replenishes the charge of the capacitor C13.

When the collector switches are open, the deposit switches are closed(during a “deposit phase”), S9 connecting the positive terminal of C12to V3 and S10 connecting the negative terminal of C12 to V4, in effectplacing the capacitor C12 in series with the capacitor C13, and thecombination of C12 and C13 in parallel with C11. In the deposit phase,some charge from the flying capacitor C12 is transferred (“deposited”)into the capacitor C11, increasing the voltage V3.

With the pumping oscillator periodically opening and closing theswitches S7 to S10 as described, the flying capacitor thus periodicallypumps charge from the voltage node V4 to the voltage node V3, increasingV3 to (ideally) double the voltage at V4 when equilibrium is reached.The voltage booster 514 operates almost without loss because only anegligible amount of power is dissipated in the switches S7 to S10. As aresult, the power (current times voltage) available for the load (theHDMI input circuit 508) at the voltage node V3 is almost equal to thepower that is delivered into the voltage node V4 which is fed by thetypical differential termination circuit 504 in series with the HDMIoutput circuit 510. Consequently, given that the amount of current drawnin the typical differential driver circuit 502 is determined by thecurrent source 13 (10 mA) in the HDMI source (Tx) and must be drawn fromV3, the current supplied from the 3.3V supply in the HDMI sink (Rx) tofeed C13 at the voltage node V4 (ultimately at one half the voltage ofV3) must necessarily be double, i.e. rise to 20 mA.

Returning now to the description of FIG. 24, we may conclude that theboost circuit provides an HDMI termination in the form of the HDMI inputcircuit 508 and an HDMI driver in the form of the HDMI output circuit510, the two circuits being interconnected via the nodes V3 and V4 (withor without the optional voltage booster 514), practically withoutrequiring external power. Only a small bias current of 0.1 mA (1% of thesignal currents) is taken from the +5V supply to set the operating pointof the circuits by controlling V4.

The differential signal recovered with the input circuit (the recoveredsignal 118) is processed by the processing block 512 into the equalizedsignal 122, which is used to drive the output circuit as describedearlier.

The processing block 512 includes analog processing circuitry (describedin FIGS. 7 to 23) which requires a certain amount of power that,depending on technology and circuit implementation could be obtainedfrom the voltage nodes V3 or V4. However, with present designconstraints it would be difficult to supply this power and at the sametime meet the HDMI specifications at the inputs and/or outputs of theboost circuit 506. Instead, according to the preferred embodiment of theinvention, power for the processing block 512 is obtained from the +5Vsupply that is provided by the HDMI source (Tx) through the HDMI cable.But because very little current (5 mA) is available from the +5V supply,it is essential to be very conserving with that power. The powerconverter 520 is used to reduce the voltage while increasing the currentavailable for the analog processing circuitry.

FIG. 27 shows a block diagram of the power converter 520. This circuitcomprises two step-down circuits 522 and 524. The first step-downcircuit 522 generates an intermediate voltage (intermediate voltage node526, +2.5V) from the +5V supply, and the second step-down circuit 524generates a +1.25V supply voltage from the intermediate voltage. The+1.25V supply voltage is then available for powering the analogprocessing circuitry in the processing block 512, i.e. the DeskewCircuit 110 and the Equalizer Circuit 112.

The first step-down circuit 522 comprises capacitors C14 and C15, andfour switches S11 to S14. The switch S11 is connected between the +5Vsupply and the positive end of the capacitor C14; the switch S13 isconnected between the positive end of the capacitor C14 and theintermediate voltage node 526 (2.5V); the negative end of the capacitorC14 is connected via the switch S14 to the common ground, and throughthe switch S12 to the intermediate node 526; and the capacitor C15 isconnected between the intermediate node 526 and the common ground.

The first step-down circuit 522 is driven by a two-phase clock signal(not shown) having two non-overlapping phases, a “charge” phase and a“discharge” phase. During the “charge” phase, the switches S11 and S12are closed while the switches S13 and S14 are open, and the capacitorC14 is thus switched into a circuit between the +5V supply and theintermediate voltage node 526. During the “discharge” phase, theswitches S11 and S12 are opened while the switches S13 and S14 areclosed, and the capacitor C14 is thus switched into a circuit that isparallel with the capacitor C15, i.e. between the intermediate voltagenode 526 and ground. The frequency of the two-phase clock signal mayconveniently be around 15 MHz, the same frequency that would also beused for pumping in the similar circuitry of the optional voltagebooster 514 (FIG. 26).

After the step-down circuit 522 has been running for a short time andhas reached equilibrium, the voltage at the intermediate voltage node526 will have risen from 0V to about one half of the input voltage of+5V, that is to 2.5 V.

The first step-down circuit 522 acts effectively as a (almost) loss-lessDC-DC converter that transforms +5V into +2.5V. The second step-downcircuit 524 comprises capacitors C16 and C17, and four switches S15 toS18. The switch S15 is connected between the intermediate voltage node526 and the positive end of the capacitor C16; the switch S17 isconnected between the positive end of the capacitor C14 and the +1.25Vsupply voltage output; the negative end of the capacitor C16 isconnected via the switch S18 to the common ground, and through theswitch S16 to the +1.25V supply voltage output; and the capacitor C17 isconnected between the +1.25V supply voltage output and the commonground.

The operation of the second step-down circuit 524 is analogous to thatof the first step-down circuit 522, using the same two-phase clocksignal for closing and opening the switches S15 to S18, to generate the+1.25V supply voltage.

The power converter 520 may thus be realized as the combination of thefirst and second step-down circuits 522 and 524, which is an (almost)loss-less DC-DC converter that transforms +5V into +1.25V.

Parameter Setup

The improved HDMI cable 20 comprising four boost circuits may bemanufactured with any of a number of different lengths of the basic(passive) HDMI cable 40. It is a further object of the invention toprovide methods for calibrating the deskew and equalization parametersto compensate for the differential skew and the frequency response ofthe cable.

As shown in FIGS. 5 and 6 above, the Parameter Memory 102 is connectedto the deskew parameter inputs 126 of each of the Differential Deskewcircuits 110, and to the equalization parameter inputs 128 of each ofthe Equalization circuits 112. The Parameter Memory 102 may be loadedwith parameter values at the time of manufacture.

The Parameter Memory 102 may be integrated within the Boost Device 100,or may be a separate device, mounted on a small Printed Circuit Board(PCB) or other carrier together with the Boost Device 100.

Three alternative methods are proposed for calibrating the parameters: aReal Time Calibration method; a Frequency Domain Calibration method; anda Time Domain Calibration method. Because the physical cable is fairlystable, it is not intended to dynamically adjust these parameters in thefield, once they have been set originally, although the Real TimeCalibration method could certainly be adapted to perform this.

In all calibration methods, access to the boost device for controllingthe calibration process (setting parameters) is provided within the“Other HDMI Signals” 54 (FIG. 5), in the form of a control buscomprising “Serial Data” (SDA) and “Serial Clock” (SCL).

FIG. 28 illustrates the improved HDMI cable 20 of FIG. 4, showingexternal connections that are available for calibrating the cable. Notethat there is no direct physical access to the Boost Device 30, and onlyexisting HDMI signals are used. The connections used in the calibrationprocesses are: 532: +5V supply and ground (2 wires); 534: fourdifferential channel inputs (8 wires); 536: four differential channeloutputs (8 wires); and 538: the control bus SDA+SCL (2 wires).

The wires of the power supply (532) and of the control bus (538) simplygo through the cable 20, and thus appear at both ends. The differentialinput and output channels (534 and 536 respectively) terminate on theboost device 30 (100) within the cable.

FIG. 29 shows a Real Time Configuration 540 used in the Real Time CableCalibration method. The Real Time Configuration 540 includes a Real TimeTest Equipment 542 and the improved HDMI cable 20 of FIG. 4, whichhowever includes an expanded boost device 544. The expanded boost device544 includes the boost device 30 (FIG. 5) and additional circuitry foranalyzing the boosted signal 124 and providing access to the control bus538.

The Real Time Test Equipment 542 includes a +5V Supply to supply powerto the cable (+5V power and ground 532); a Data Pattern Generator forgenerating HDMI-conforming differential data and clock signals to feedthe differential channel inputs 534, and a Control Computer (PC) tocontrol the data patterns to be output by the Data Pattern Generator,and to communicate with the expanded boost device 544 in the cable overthe control bus (SDA+SDL) 538. A termination device “Term” thatcomprises a set of typical differential termination circuits 504 (FIG.24) is connected to the differential channel outputs 536.

To calibrate the cable (each cable is individually calibrated atproduction) the Real Time Calibration method includes the followingsteps:

-   a control program in the PC instructs the Data Pattern Generator to    send HDMI data patterns into the differential channel inputs 534 of    the cable;-   the control program in the PC uses the control bus 538 to send    deskew and equalization parameters to the expanded boost device 544;-   the expanded boost device 544 performs the deskew and equalization    steps as determined by the set parameters;-   the expanded boost device 544 analyzes the quality of the deskewed    and equalized signal;-   the expanded boost device 544 reports the quality result to the PC    over the control bus 538;-   the preceding steps are repeated for each differential channel and    with different parameters;-   the best settings are determined and permanently, set into the    parameter memory 102 within the expanded boost device 544.

For an additional check to verify the proper operation of the calibratedcable, a built-in self test (BIST) may be included in the expanded boostdevice 544 in which the reception of a known pattern sent from the DataPattern Generator into the differential channels of the cable isverified in the expanded boost device 544.

FIG. 30 shows a simplified block diagram of the expanded boost device544 including the boost device 30 of FIG. 5, a Control Interface 546,and a performance analysis circuit 548. Only a representative one of thefour channel boost circuits 100 is shown in the FIG. 30, it beingunderstood that each of the three differential TMDS channels and thedifferential clock channel are processed by a respective channel boostcircuits 100.

The Control Interface 546 communicates with the Real Time Test Equipment542 over the control bus 538, and with the parameter memory 102 (in theboost device 30) over a parameter setup link 550.

The performance analysis circuit 548 is only active (powered up undercontrol of the Control Interface 546) when the expanded boost device 544is being calibrated.

The performance analysis circuit 548 includes aDifferential-to-Single-Ended block 552, a Linear Phase Compensator 554,an Oversampling and Reclocking block 556, and a Training Function block558. An output of the Training Function block 558 is connected to aninput of the Control Interface 546 over a control link 560. Two optionaloutputs (parameter links 561) of the Training Function block 558 areconnected to the deskew and equalization parameter inputs 126 and 128 ofthe channel boost circuit 100, bypassing the Parameter Memory 102.

Not shown in FIG. 30 is a conventional clock recovery circuit whichrecovers the clock from any of the differential channels, and generatesa multiphase clock signal (clock phases PH0 to PH23). The generation ofthe multiphase clock signal may be accomplished with a phase locked loopusing any of a number of known techniques to generate multiple phases ofa clock, and is not described in detail here.

While each of the four channel boost circuits 100 is being calibrated bythe Real Time Cable Calibration method, its “boosted signal” pair 124 istapped and connected to the performance analysis circuit 548.

Note that a single common performance analysis circuit 548 may be sharedfor calibrating the four channel boost circuits 100 sequentially.Alternatively, a plurality of performance analysis circuits 548 may beincluded in the expanded boost device 544 which would allow the channelboost circuits 100 to be calibrated in parallel.

In the performance analysis circuit 548 this differential signal isconnected to the Differential-to-Single-Ended block 552 which convertsthe boosted signal 124 into a single-ended signal 562 that is input tothe Linear Phase Compensator 554 which also receives the PH0 phase ofthe multiphase clock signal, and produces as output a phase alignedsignal 564 (that is, a preprocessed data signal).

The Oversampling and Reclocking block 556 receives the phase alignedsignal 564 as well as all 24 phases (PH0 to PH23) of the multiphaseclock signal, to generate a 24-sample digital samples signal 566 whichis then input to the Training Function block 558.

Analog Phase Recovery (Linear Phase Compensator 554)

After being converted to the single-ended signal 562 in theDifferential-to-Single-Ended block 552, the data is ready to be sampled(converted into a digital signal). The problem, however, is that thephase of the data relative to the sampling clock is unknown. When thisphase relationship is unknown, there is a danger of sampling during datatransitions and misinterpreting the data in the data stream. To definethe phase relationship between the on-board clock (PH0 of themulti-phase clock) and the data (the single ended signal 562), an AnalogPhase detector (within the Linear Phase Compensator 554) is used. Thefrequency of the data and the recovered clock are equivalent because thetimings in both are derived from the same source, that is, thetransmitted clock, so there is no need for frequency adjustment. TheLinear Phase Compensator 554 employs a scheme similar to that describedin the paper entitled “A 10-Gb/s Clock Recovery Circuit with LinearPhase Detector and Coupled Two-stage Ring Oscillator” by Afshin Rezayeeand Ken Martin. This paper, which is incorporated herein by reference,was published at the European Solid State Circuits Conference (SSCIRC)in Florence, Italy in the year 2002, pp. 419-422.

In this phase detection scheme of Rezayee and Martin, a window in timeis generated around rising edges in the data stream. The phase detectoris only enabled within this window. The window is of such a length thatone clock edge is guaranteed to be present, but only one. In the Rezayee& Martin implementation, the clock and data are locked such that clockedges occur in the middle of the data bits. This allows the alignedclock to sample in a region where the data is stable.

In the implementation of the phase detector circuit described herein,the Linear Phase Compensator 554 aligns the clock and data edges. Theresulting phase aligned data signal (the phase aligned signal 564) issubsequently over-sampled in a separate circuit block (The Oversamplingand Reclocking block 556) before the bit value may be determined.

FIG. 31 shows a block diagram of an exemplary implementation of theLinear Phase Compensator 554. The Linear Phase Compensator 554comprises:

-   a Programmable Analogue Delay 568 having a data input (Din) and a    control input (Cin); and an Analog Phase Detector (APD) 570, which    includes:—a Window Generator 572;-   a Phase Detector 574 having a clock input “Ck”, a data input “Data”,    and an enable input EN;-   and a Charge Pump 576 with inputs “Up” and “Down”, and including a    capacitor C18.

The inputs to the Linear Phase Compensator 554 are the data signal (thesingle ended signal 562), and the clock signal (the PH0 of the recoveredmulti-phase clock). The data signal is connected to the data input (Din)of the Programmable Analogue Delay 568, the output of which is the phasealigned signal 564 (the preprocessed data signal). This signal (564) isfurther connected to the input of the Window Generator 572 the output ofwhich is connected to the enable input “EN” of the Phase Detector 574.The clock input “Ck” of the Phase Detector 574 receives the phase 0(PH0) of the multi phase clock signal. The outputs of the Phase Detector574 drive the “Up” and “Down” inputs of the Charge Pump 576. The outputof the Charge Pump 576 is an analog control signal, connected to thecontrol input Cin of the Programmable Analogue Delay 568.

The Window Generator 572 detects positive edges on the input data andgenerates the enable (EN) signal for the Phase Detector 574, of durationguaranteed to contain an edge of the clock to which the data is to belocked.

The Phase Detector 574, uses the enable signal (EN), supplied by theWindow Generator 572, to compare the phases of the “Data” and “Ck”signals during the length of the enable signal (EN). The outputs of thephase detector control the Programmable Analogue Delay 568, by means ofthe Charge Pump 576, which is a control voltage generator, generating acontrol voltage by charging the capacitor C18 or by other suitablemeans.

The Programmable Analogue Delay 568 takes the control signal from thephase detector 570 (the control input “Cin”) and delays the data signalby a programmable amount to align it with the clock signal. The outputof the Programmable Analogue Delay 568 is thus the phase aligned signal564.

This Linear Phase Compensator 554 works robustly in the presence of ISI(Inter-Symbol Interference) and jitter and aligns the on-board clockedges with the substantially “ideal” data transition points in the datachannels.

Oversampling

The phase aligned (data) signal 564 is a rail-to-rail analog signal thatmay still contain Inter Symbol Interference (ISI), distortion, noise,and other impairments. In the Oversampling and Reclocking block 556(FIG. 30), this signal is effectively sampled at a rate 12 times theclock rate of the signal, i.e. during each bit period the data signal issampled at 12 evenly spaced intervals, to generate 12 digital samples.Because of the high speed of the signal (typically 1.65 Gbs) it is notpractical to actually sample the signal with a 12-times higher clocksignal. Instead, the same effect is achieved by sampling the signal with12 evenly spaced phases of the clock signal, each clock phase generatinga digital sample, thus 12 samples representing one data bit. In thepresent embodiment, 24 clock phases (PH0 to PH23 of the multiphase clocksignal) are used to capture not only one data bit in 12 sampling phases,but also the trailing half of the previous data bit in 6 sampling phasesand the leading half of the next data bit in another 6 sampling phases(conventional digital register logic and pipelining is used to thus lookinto the “future”).

Because of the oversampling, the term “bit” might become ambiguous. Theterms “bit”, “primary data bit”, and “bit-clock period” will be used todenote the nominal 1.6 Gbs data bits and their period; “sample” and“sample bit” to denote one of the 12 samples per bit-clock period; and“24-sample word” to denote the ensemble of 24 samples, as described.

Thus, the Oversampling and Reclocking block 556 generates 24 samples (a“24-sample word”) at the bit-clock rate, by outputting the 24-sampledigital samples signal 566.

FIG. 32 illustrates data phase shifting in the Programmable AnalogueDelay 568 of FIG. 31, and oversampling in the Oversampling andReclocking block 556 of FIG. 30. The diagram 600 in FIG. 32 shows anexemplary waveform 602, a delayed waveform 604, a set of sampling clocks606, a 24-sample word 608, and a scale indicating a bit-period andprevious and next bits.

The exemplary waveform 602 represents an example of the single endedsignal 562 (FIG. 30) before phase alignment. Note that the signalappears to be a “1” bit with some distortion (noise or ISI) near theone-zero transition, and it is not aligned with the indicatedbit-period. The delayed waveform 604 represents the corresponding phasealigned signal 564 after delay through the Linear Phase Compensator 554.Note that the signal is now approximately aligned with the indicatedbit-period, but still includes the distortion. This signal is sampledwith the 24 phases of the multiphase clock (PH0 to PH23) as indicated bythe set of sampling clocks 606 in the Oversampling and Reclocking block556, resulting in the 24-sample word 608. The 24-sample word 608includes six samples (000000) from the previous bit period, twelvesamples (111111111100) from the Bit-period and another six samples(000000) from the next bit period.

The 24-sample word 608 is output by the Oversampling and Reclockingblock 556 as the 24-sample digital samples signal 566 to the TrainingFunction 558.

Eye Quality Determination

The Training Function 558 (FIG. 30) may provide feedback to the RealTime Test Equipment 542 by evaluating the 24-sample digital samplessignal 566, which is a stream of 24-sample words such as illustrated inthe 24-sample word 608 of FIG. 32. In this way, the Time Domain TestEquipment 542 may be able to tune the adjustable parameters of thechannel boost circuit 100 that is presently being calibrated.

The approach taken in the preferred embodiment of the invention is tosystematically go through each of the possible permutations of settingsof these parameters; observe and measure the quality of the preprocessedsignal (the single ended signal 562 that is oversampled as the 24-sampledigital samples signal 566) to obtain a quality measure in the form of a“Quality Number”; and retain the settings that yield the best QualityNumber in the parameter memory 102 (FIG. 30).

The deskew and equalizer settings may include (actual values in theexample embodiment are shown in brackets, based on the bit oversamplingfactor of 12):

-   settings of differential delay compensation (7 values, ranging from    about 0 to approximately 360 psec);-   insertion of the differential delay in the positive or negative    polarity signal (positive or negative); and-   up to 32 frequency response (cable) equalization settings.

Note that the phase offset between the bit-clock and the data-bit is notof interest here, being independently and automatically adjusted by theLinear Phase Compensator 554. The phase aligned data signal 564 will befairly accurate in phase, that is centering the nominal bit-period onthe middle twelve samples of the 24-sample word, provided the deskew andequalizer are within the vicinity of the optimal settings. If they arenot, it does not matter if the data/clock phase alignment is suboptimal.

Implementation of the Training Function 558

Although the Real Time Calibration method could be conducted under stepby step control through the PC as described above (FIG. 30), it may beadvantageous to allow the Training Function 558 to bypass the ParameterMemory 102 and perform repetitive steps of setting trial values of theparameters (126 and 128) autonomously, and only report the final resultfor each channel to the PC which may then load the “best” settings intothe Parameter Memory 102.

Alternatively, the PC may be used only to start the Real TimeCalibration, the final results (the “best setting”) being autonomouslyloaded into the parameter memory without intervention by the PC.

FIG. 33 shows a simplified block diagram of the preferred embodiment 700of the Training Function 558. The Training Function 700 includes thefollowing blocks:

-   a Bit Length Detection block 702;-   a set of Length-i counters (i=5 to 12), designated by reference    numerals 704 to 718;-   a Bit Quality Calculator 720 including a Best Quality Number    register 722;-   a Best Settings Memory 724 having inputs D and W, and an output Q;-   a write-enable gate EN 726;-   a Current Settings Memory 728;-   an Evaluation Run Control block 730; and-   a selector MUX 732.

The inputs to the Training Function 700 are the 24-sample digitalsamples signal 566 that is connected to the Bit Length Detection block702, and the clock (PH0 of the multiphase clock signal). The output ofthe Bit Length Detection block 702 is a set 734 of count-enable signals,one count-enable signal connected to each of the Length-i counters 704to 718. The outputs of each of the Length-i counters 704 to 718 provideinputs to the Bit Quality Calculator 720. The Bit Quality Calculator 720in turn is connected with a “save best settings enable” control signal736 to the write-enable gate EN 726. The other input of the write-enablegate EN 726 receives an “end-of-calculation” signal 738 from theEvaluation Run Control block 730. The output of the write-enable gate EN726 is connected to the write control input “W” of the Best SettingsMemory 724. The output Q of the Best Settings Memory 724 sends amulti-bit “best settings” signal 740 which is a digital control wordindicative of deskew and equalization settings values. The “bestsettings” signal 740 is connected to one of the two data inputs of theselector MUX 732 whose other data input receives a similar data word,i.e. a “current settings” signal 742 from the Current Settings Memory728. The “current settings” signal 742 is also applied to the data inputD of the Best Settings Memory 724. The outputs of the Evaluation RunControl block 730 include the “end-of-calculation” signal 738 connectedto the write-enable gate EN 726 (already mentioned above), and an“end-of-search” signal 744 connected to the select input of the selectorMUX 732. The output of the selector MUX 732 (outputting either the“current settings” 742 or the “best settings” 740 depending on the stateof the “end-of-search” signal 744) is split into the deskew parameters126 and the equalization parameters 128 that are sent out on theparameter links 561, see FIG. 30.

The Training Function 700 is further connected by the control link 560and the control interface 546 (FIG. 30) to the PC in the Real Time TestEquipment 542 (FIG. 29), for the purpose of starting the elevation runcontrol 730 and reporting the “best settings” signal 740 or the “currentsettings” signal 742 as may be required by the control program in thePC.

The overall operation of the Training Function 700 is controlled by theEvaluation Run Control block 730 which, briefly noted, allows the testto run (an “evaluation run”) for a specific period of time(corresponding to a specific number N of received data bits) at each ofthe predefined sets of parameter settings (“current settings”). Each“evaluation run” of the Training Function 700 runs for a durationequivalent to the N primary data bits (an observation period of “IN”bits). A “training run” is the sequence of “evaluation runs”, each witha different set of “current settings”. The purpose of the “TrainingFunction” is to select the permutation of deskew and equalizationsettings that gives the “best” (highest) Quality Number, and reportthese settings to the PC over the control link 560 and the control bus538, as the calibration result for subsequent loading into the parametermemory 102 by the PC. The Training Function may be invoked (started) bya trigger received from the PC over the control link 560. The operationof the “training run” is further described with the help of a flow chart(FIG. 34 below). The functions of the individual blocks of the TrainingFunction 700 shown in FIG. 33 are briefly explained first.

The Bit Length Detection block 702 receives the 24-sample digitalsamples signal 234 indicating an oversampled received bit (nominally inthe middle 12 samples) and samples of adjacent bits, as described above(FIG. 32), and treating it as a digital word of 24 bits (samples); anddetects within each such digital word clusters (runs) of adjacent “1 s”,bracketed by at least one “0” sample at each end. For example the24-sample word 608 of FIG. 32 “000000111111111100000000” contains a runof ten “1 s” samples. The function of the Bit Length Detection block 702is to classify each arriving 24-sample word 608 by the lengths of the “1s” run (if any) contained in it and increment the corresponding Length-icounter (704 to 718) accordingly. In the example above the Length-10counter 714 would be incremented.

Note that there are no counters for lengths below 5 or above 15; theselengths are ignored.

The Length-i counters 704 to 718 thus, record and accumulate the numberof occurrences of the corresponding run lengths of “1 s” in the streamof 24-sample words in the digital samples signal 234, for eachevaluation run.

At the end of each evaluation run, the outputs of the Length-i counters704 to 718 are fed into the Bit Quality Calculator 720, which computes aQuality Number from the ensemble of accumulated length counts accordingto a heuristic algorithm. Recall that the purpose of “training” theanalog front end is to find the “best settings”, that is the settingswhich results in the most appropriate equalization setting (see theEqualization block 206, FIGS. 2 and 22) and which “optimally” removesany differential skew that might exist by adjusting the DifferentialDeskew 204. An ideal data signal of alternating “1 s” and “0 s”, thatwas perfectly phase aligned (see Linear Phase Compensator 210, FIG. 23)would after oversampling result in successive 24-sample words of:

000000111111111111000000

111111000000000000111111 . . .

and result in high counts for the run length 12. The runs of length 6would not be counted, as only contiguous runs of “1” samples with “0”samples on either side of the run are counted. Thus, the six sampleslocated at the end of the window are not counted—they are part of a bitthat was or will be counted in the previous or subsequent bit periodrespectively.

If the signal shape was perfect (twelve “1” samples per bit) but phasealignment was skewed by one or a few samples, the result would be thatthe same high counts for the run length 12 would be recorded. If thesignal was distorted (imperfect differential deskewing, high ISI, ornon-optimal equalization setting), other lengths may be recorded.

At the end of an evaluation run the Quality Number is computed by theBit Quality Calculator 720, by multiplying the contents of each Length-icounter 704 to 718, with a length specific weight, and summing theproducts:

for i=5 to 12, Bit Quality Number=SUM(Length.sub.i count×Weight.sub.i)

The following set of weights have been used in the embodiment of theinvention, but other weights may also give good results:

-   Weight.sub.5=−2-   Weight.sub.6=−2-   Weight.sub.7=−1-   Weight.sub.8=1-   Weight.sub.9=1-   Weight.sub.10=2-   Weight.sub.11=4-   Weight.sub.12=8

The selected weight numbers suggest, as may be expected, that a runlength of 12 being indicative of a perfect pulse has the highest weight,while run lengths below 8 may be indicative of severe distortion,resulting in a negative contribution to the Bit Quality Number.

The Bit Quality Number from each evaluation run with a particular set ofsettings (the current settings) is compared with the currently storedBest Quality Number (in the register 722). If it exceeds the previousBest Quality Number, the Best Quality Number 722 is updated with thehigher number, and the current settings is saved in the Best SettingsMemory 724. This functionality is indicated in FIG. 33 where the outputof the Bit Quality Calculator 720 (the “save best settings enable”control signal 736) is ANDed with the “end-of-calculation” signal 738from the Evaluation Run Control block 730 in the write-enable gate EN726 to generate a write signal (“W” input) for the Best Settings Memory724 while at the same time, the current settings (the “current settings”signal 742 from the Current Settings Memory 728) is presented at thedata input “D” of the Best Settings Memory 724, causing it to store thecurrent settings.

If on the other hand with a given current settings, a Bit Quality Numberis obtained that is not higher than the Best Quality Number alreadystored in the register 722, the write-enable gate EN 726 is not enabled,and the current settings is not stored in the Best Settings Memory 724.

The Evaluation Run Control block 730, for each evaluation run, chooses acurrent settings permutation and stores it in the Current SettingsMemory 728 for the duration of the each evaluation run. During eachevaluation run, the “current settings” 742 are fed through the selectorMUX 732 to provide the deskew and equalization parameters (126 and 128over the parameter links 561) to the Differential Deskew andEqualization blocks (110 and 112 respectively).

After all permutations are exhausted, that is at the end of the“training run”, the “end-of-search” signal 744 is asserted by theEvaluation Run Control block 730 which then causes the selector MUX 732to send the “best settings” into the deskew and equalization parametersignals (126 and 128 over the parameter links 561).

The number of received data bits N for which each evaluation run isheld, may be determined under control of the PC, which also determinesthe data pattern to be sent by the Data Pattern Generator duringcalibration. The number N may range from about 256 to 10000 depending onthe length of the cable and the nature of the data pattern.

Due to present technology limitations, the blocks 702 to 718 of theTraining Function circuit 700 are duplicated (duplication not shown inFIG. 33). Each of these blocks operates at half speed, processing the24-sample digital samples signal 566 for alternate received data bitswith the Bit Quality Number simply computed at the end of eachevaluation run from the contents of the Length-i counters of both setsof counters. Thus in effect, a total of 2N bits are processed for eachevaluation run.

Alternative implementations of the Training Function 558 are alsoenvisaged which may differ in the details from the embodiment 700. Forexample, the number of clock phases for oversampling the received datasignal may be less or more than 24, and the window of oversampling mayinclude at least one bit period (the middle samples), but be narrower orwider with respect to adjacent bits. Instead of counting run lengths of“1” samples, run lengths of “0” samples may be accumulated, anddifferent weightings may be applied to the run length counts. These andother variations that may occur to skilled persons are included in thescope of the invention.

FIG. 34 shows a high level flow chart of a training run 800, depictingthe operation of the Training Function 558 (corresponding to theembodiment 700 of FIG. 33). The training run 800 is a finite processthat may be invoked to run from “Start” to “Finish” through a number ofsteps that are either actions or logic decisions:

-   802: “Reset the best Quality Number (bestQN)”;-   804: “Get the first current Settings”;-   806: “Do an Evaluation run”;-   808: “Compute a Quality Number (QN)”;-   810: “Is the computed Quality Number greater than the best Quality    Number (QN>bestQN)?”, Yes or No;-   812: “Set the best Settings to the current Settings, and set the    best Quality Number to the computed Quality Number    (bestsettings:=currentSettings; bestQN:=QN);-   814: “Is Training Finished?”, Yes or No;-   816: “Get the next current Settings”; and-   818: “Send the best Settings to the PC”.

The current Settings refers to the parameters that may be controlled,that is the differential deskew and equalization parameters 126 and 128,FIG. 30. At the start of the training run, a stored variable “bestQuality Number” (bestQN) is initialized (“reset bestQN” 802) and a firstset of the parameters is created (“Get first currentSettings” 804). Thisis followed by a loop over the steps 806 (“Do an Evaluation run”) to 816(“Get the next current Settings”) which is executed until all settings(permutations of the parameters) have been exhausted and training isfinished, as indicated by the step 814 (“Is Training Finished?”). Thetraining run 800 ends with the step 818 (“Send the best Settings to thePC”).

Within the loop (steps 806 to 816), the step 806 (“Do Evaluation run”)is followed by the step 808 (“Compute a Quality Number”) which computesthe Quality Number from the results of the evaluation run. This step 808may be performed by the Bit Quality Calculator 720 of FIG. 33, forexample. In the next step 810 “Is the computed Quality Number greaterthan the best Quality”, a comparison is made between the last computedquality number (QN) and the stored “best Quality Number” (bestQN). If QNis greater than bestQN then the current settings is assigned and storedin a variable “best Settings”, and also the stored variable “bestQN” isupdated with the last computed QN (the step 812). In the step 814 “IsTraining finished?”, it is determined if all valid permutations of theparameters have been evaluated. If training is NOT finished, the nextpermutation is created in the step 816 “Get next current settings”, andthe loop continues with the evaluation run (step 806). If there are nomore permutations to evaluate, training is finished (“Yes” in the step814 “Is Training finished?”), the current settings are abandoned, andthe best Settings are sent to the PC in the step 818, before thetraining run 800 exits.

The Evaluation run of the step 806 is further detailed in a subroutineflow chart of an exemplary evaluation run method 900 that is shown inFIG. 35. The evaluation run 900 runs from “Enter” to “Return” through anumber of steps that are either actions or logic decisions:

-   902: “Send the current Settings to the differential deskew and    equalization blocks”;-   904: “Reset the Length[i] counters”;-   906: “Get the next oversampled bit”;-   908: “Compute the run lengths (RL)”;-   910: “for each i for which RL[i] is not 0, increment the Length[i]    counter”; and-   912: “Is Evaluation run finished?”.

The current settings (see the flow chart of the “Training run” 800) aresent to the differential deskew block 110 and the equalization block 112(over the parameter links 561) in the step 902, and remain constant forthe duration of the evaluation run 900. The run is initialized byresetting all Length counters to 0 (zero) in the step 904. Thesecounters correspond to the Length counters 704 to 718 of the embodiment700 (FIG. 33). Only counters for i=5 to 15 (selected run length 5 to 15)are provided in the present embodiment of the invention, but otherranges may be used.

The next oversampled data bit and adjacent samples is obtained in thestep 906. This “Oversample” corresponds to the 24-sample digital samplessignal 566 of the earlier description (FIGS. 30). In the next step 908(“Compute run lengths”), the received oversample is analyzed todetermine run lengths of “is” as described earlier (the Bit LengthDetection 702, FIG. 33). This step produces an indication for each runlength (only run lengths of 5 to 15 are covered) that is found in theoversample. In the next step 910 (“for each i for which RL[i] is not 0,increment the Length[i] counter”), each Length[i] counter for which arun length was indicated in the previous step is incremented.

The end of the evaluation run is indicated in the step 912 “IsEvaluation run finished?” if a sufficient number of data bits(oversamples) have been processed, in other words, a simple loop countis maintained, the evaluation run exits, that is it returns to the nextstep 808 in the training run 800 where the contents of the Lengthcounters are converted into the Quality Number.

Alternative implementations of the Real Time Cable Calibration methodare also envisaged which may differ in the details from the embodiment540 with the embodiment 700 of the training function. For example, somefunctions of the training function such as the bit quality calculationcould be performed in the PC instead of within the expanded boost device544, which would require the contents of the Length-i counters (704 to718) to be periodically communicated from the expanded boost device 544to the PC over the control bus. These and other variations that mayoccur to skilled persons are included in the scope of the invention.

FIG. 36 shows a generic test set up 1000 for the Frequency Domain andthe Time Domain Calibration methods. The generic test set up 1000includes the improved HDMI (High-Definition Multi-Media Interface) cable20 (see FIG. 28), a PC 1002, and a test equipment 1004 that is either aVNA (Vector Network Analyzer) or a TDR (Time Domain Reflectometer). ThePC 1002 is attached to the control bus (SDA+SCL) 538 of the cable. Thetest equipment 1004 is connected to the differential channels at bothends of the cable, that is the four differential channel inputs (8wires) 534 and the four differential channel outputs (8 wires) 536.

The test equipment 1004 is controlled by the PC 1002 over a standardPC-interface 1006 to send stimulus signals into the cable inputs (534)and to receive measurement results from the cable outputs (536). Theresults are passed back to the PC over the standard PC-interface 1006for evaluation.

It is possible with the test equipment 1004 being either a VNA or a TDRto obtain both frequency attenuation and delay characteristics of thecable, although well-known mathematical transformations are required toconvert between the frequency and time domain results obtained with theVNA or the TDR respectively.

FIG. 37 shows a simplified high level flow chart of an calibrationmethod 1100 that may be used with the generic test set up 1000 incalibrating the Boost Device 30 in the improved HDMI cable 20, includinga number of steps:

-   1102: “Select a first deskew parameter setting”;-   1104: “Measure differential skew”;-   1106: “Is skew acceptable?” (YES: goto step 1110, NO: goto step    1108);-   1108: “Change deskew parameter setting”;-   1110: “Select a first equalizer parameter setting”;-   1112: “Measure attenuation”;-   1114: “Is attenuation acceptable?” (YES: goto finish, NO: goto step    1116); and-   1116: “Change equalizer parameter setting”.

The calibration method 1100 includes two loops, a first loop (the steps1104 to 1108) for setting the deskew parameter, and a second loop (thesteps 1112 to 1116) for setting the equalizer parameter. The calibrationmethod starts with an (arbitrary) first deskew parameter setting (thestep 1102), in which the PC 1002 loads a first deskew setting into theparameter memory 102 of the boost device 30 (FIG. 36).

In the step 1104, the end-to-end differential skew of the differentialchannel that is being calibrated (from the input 534 to the output 536through the improved HDMI cable 20 including the boost device 30) ismeasured by the test equipment 1004 and reported to the PC 1002.

In the step 1106, the measured result is processed in the PC, andcompared with a skew threshold set for the test, and with previous testresults. If the result proves to be acceptable, below the skew threshold(and ideally minimized), the calibration method proceeds to the step1110, otherwise the deskew parameter setting is changed (in the step1108), and the calibration method loops back to the step 1104.

In the unlikely event that an acceptable differential skew measurementis not found after all deskew settings have been tried, the cable isdeemed to be defective.

In the step 1110, the calibration method continues with an (arbitrary)first equalizer parameter setting, in which the PC 1002 loads a firstequalizer setting into the parameter memory 102 of the boost device 30(FIG. 36). It may also be desirable to set the same equalizer values forall cable from the same lot, in that all cable in the same lot will havesimilar characteristics, thus saving time in production.

In the step 1112, the end-to-end attenuation of the differential channelthat is being calibrated (from the input 534 to the output 536 throughthe improved HDMI cable 20 including the boost device 30) is measured bythe test equipment 1004 and reported to the PC 1002. In order to ensurea near optimal setting of the equalization parameters, it is necessaryto measure attenuation at frequencies up to about the frequency of thefastest signal to be transmitted in the differential channel to up toabout a frequency of 2/(bit time)−4(bit time) of the data.

In the step 1114, the measured result (the measured gain figures for allfrequencies in the range of interest) is processed in the PC 1002, andcompared with a requirement of being within a predetermined range, thatis close to 0 db or greater (a minimum requirement of the HDMIspecification), and less than a predetermined limit. If the resultproves to be acceptable, i.e. within the predetermined range, thecalibration method finishes, otherwise the equalizer parameter settingis changed (in the step 1116), and the calibration method loops back tothe step 1112.

In the unlikely event that an acceptable attenuation (gain) measurementis not found after all equalizer settings have been tried, the cable isdeemed to be defective.

This calibration method has to be successfully run for each of the fourdifferential channels of the cable, after which the cable is consideredto be calibrated and meeting HDMI specifications.

FIG. 38 shows an alternative embodiment of the invention, in the form ofa modified improved HDMI cable 1200. The modified improved HDMI cable1200 comprises the basic HDMI cable 40 (unchanged from its use in theimproved HDMI cable 20); a small printed circuit board (PCB) 1202; aconnector 1204; and a modified boost device 1206 mounted on the PCB1202.

The PCB 1202 provides physical support for the modified boost device1206, as well as connectivity (PCB tracks) to the conductors of thebasic HDMI cable.

The modified boost device 1206 is based on the boost device 30, withadditional inputs provided.

It may be recalled that the boost device 30 provides a number offunctions, including the differential deskew circuit 110 (FIG. 5) foradjusting an existing time skew of the polarities of differentialsignals propagating through the basic HDMI cable 40.

In the boost device 30, each of the polarities of each of thedifferential signals (the HDMI inputs 50, FIG. 6) is directly connectedto the boost device 30.

In the modified improved HDMI cable 1200, each of the polarities of eachof the differential signals is connected to two or more (preferablythree) selectable inputs of the modified boost device 1206 throughtracks of the PCB 1202 as shown in FIG. 38. For clarity, only thepositive polarity of an example one of the HDMI inputs 50, connected tothree inputs of the modified boost device 1206 is illustrated:

-   -   a single polarity signal lead 1208 is directly, or via a short        PCB track, connected from the basic cable 40 to a first input        terminal 1210 of the modified boost device 1206;    -   the first input terminal 1210 is connected through a first PCB        track 1212 to a second input terminal 1214 of the modified boost        device 1206; and    -   the second input terminal 1214 is connected through a second PCB        track 1216 to a third input terminal 1218 of the modified boost        device 1206.

The negative polarity of the example one of the HDMI inputs 50, and bothpolarities of the other HDMI inputs 50 as well, are routed similarlythrough short PCB tracks, each to a separate set of three terminals ofthe modified boost device 1206.

The PCB tracks 1212 and 1216 (shown symbolically and not to scale) aredesigned to each provide a small delay of the signal arriving from thebasic HDMI cable 40. The modified boost device 1206 thus receives threecopies of the same signal, each delayed by a small amount (preferably100 picoseconds, corresponding to approximately 2 cm of PCB track), atthe three input terminals 1210, 1214, and 1218. In the modified boostdevice 1206 any one of the three signals from any of the input terminalscan be independently selected for each polarity of each of thedifferential HDMI inputs. After selection, the signals are processed inthe modified boost device 1206 in the same manner as was described forthe boost device 30 above.

In this way, deskewing of the differential signals can be achieved bycoarse and fine adjustments. The coarse adjustment is done by selectingone or two PCB delays of either polarity for each of the differentialsignals. The fine adjustment is done by adjusting the adjustable delay300 of the Differential Deskew circuit 110 (FIG. 7). Thus a wider rangeof deskewing can be achieved, or conversely, less on-chip circuitry(fewer delay stages 306) need be provided in the modified boost device1206, compared with the boost device 30.

FIG. 39 shows a typical (one of four) modified boost circuit 100A of themodified boost device 1206 analogous to the boost circuit 100 of FIGS. 5and 6, in which corresponding elements are shown with the same referencenumerals. The modified boost circuit 100A includes the HDMI Inputcircuit 106, the Differential Deskew circuit 110, the Equalizationcircuit 112, and the HDMI Output circuit 108.

Also shown in FIG. 39 are the raw signal input (pair) 116 (see FIG. 6)including positive and negative polarities (V+ and V− respectively), andthe PCB tracks (delay elements) 1212 and 1216 that connect the positivepolarity (V+) to the three input terminals 1210, 1214, and 1218 of themodified boost device 1206 as shown in FIG. 38.

The negative polarity (V+) of the raw signal input (pair) 116 issimilarly connected to three input terminals.

In addition, the modified boost circuit 100A includes two input selectorcircuits 1220 and 1220. The input to the HDMI Input circuit 106 is adelayed raw input signal (pair) 116A, which is the original raw inputsignal (pair) 116 after passing sequentially through the delay elementsformed by the PCB tracks (1212 and 1216 in the positive polaritysignals, and equivalent delays in the negative polarity). The HDMI Inputcircuit 106 functions as the termination of the HDMI cable. Theundelayed positive polarity V+ of the raw input signal 116 and itsdelayed versions (input terminals 1210, 1214, and 1218) are input to theinput selector circuit 1220, and analogously for the negative polarityV− into the input selector circuit 1222. A “selected recovered signal”(pair) 118A, equivalent to the “recovered signal” (pair) 118 of theboost circuit 100 is generated by the input selector circuits 1220 and1222 and input to the Differential Deskew circuit 110. The “selectedrecovered signal” (pair) 118A may already be partially deskewed byselecting appropriate settings of the input selector circuits 1220 and1222.

The remaining circuitry of the modified boost circuit 100A is unchangedfrom the boost circuit 100: the Differential Deskew circuit 110 outputsthe “deskewed signal” (pair) 120 that is input to the Equalizationcircuit 112; the Equalization circuit 112 outputs the “equalized signal”pair 122 that is input to the HDMI Output circuit 108; and finally, theHDMI Output circuit 108 outputs the “boosted signal” (pair) 124 that isone of the HDMI Outputs 52 (FIG. 5).

As indicated above, the implementation of the Differential Deskewcircuit 110 may remain unchanged (e.g. having eight on-chip delay stages306, see FIG. 8), or it may include fewer (for example three) delaystages 306, thus conserving on-chip area. The control of the inputselector circuits 1220 and 1222 may be handled along with the control ofthe analog selector stage 308 (FIG. 8), to generate a correspondingrange of adjustable delay that is a combination of the adjustable delay300 (FIG. 8) and the delay provided by the selected PCB tracks.

As an example, with two PCB track delays of 100 psec each, and threeon-chip delay stages of 25 psec each, a delay range of 0 to 275 psec, insteps of 25 psec may be achieved with the modified boost device 1206.Other combination, more or fewer selectable PC track delays, and more orfewer on-chip delay stages, and longer or shorter delay increments maybe readily designed as may be required depending on the type and range(length) of HDMI cable.

Although embodiments of the invention have been described in detail, itwill be apparent to one skilled in the art that variations andmodifications to the embodiments may be made within the scope of thefollowing claims.

What is claimed is:
 1. A cable for connecting a transmitting data sourcedevice to a receiving data sink device carrying differential signalsincluding: a boost device for boosting at least one of the differentialsignals, the boost device comprising: an input circuit for receiving araw differential signal from the data source device and outputting arecovered signal; a deskew circuit with first adjustable parameters forprocessing the recovered signal into a deskewed signal; an equalizercircuit with second adjustable parameters for processing the deskewedsignal into an equalized signal; and an output circuit for amplifyingthe equalized signal into a boosted signal and sending the boostedsignal to the data sink device.
 2. The cable as described in claim 1,the boost device further including a parameter memory for retaining thefirst and second adjustable parameters.
 3. The cable as described inclaim 2, wherein the cable further includes a control bus, and whereinthe parameter memory is accessible from said control bus.
 4. The cableas described in claim 1, wherein the deskew circuit is an analogdifferential deskew circuit for adjusting an existing time skew of twopolarities of the differential signal by changing the first adjustableparameters.
 5. The cable as described in claim 4, wherein the equalizercircuit comprises a circuit for adjusting a frequency response of thedeskewed signal by changing the second adjustable parameters to producethe equalized signal.
 6. The cable as described in claim 5, wherein theequalizer circuit has at least two settings of the second adjustableparameters for adjusting the frequency response.
 7. The cable asdescribed in claim 4, wherein the analog differential deskew circuitcomprises: a number of delay units arranged sequentially; an analogselector, selecting a composite delay resulting from the delay unitsthat are selected by the analog selector; and analog switches insertingthe composite delay into the polarities of the differential signal. 8.The cable as described in claim 7, wherein the analog switches areinserting the composite delay into one or the other polarity of thedifferential signal.
 9. The cable as described in claim 8, wherein eachof the analog delay units has a gain which is substantially equal to1.0.
 10. The cable as described in claim 18, wherein each of the analogdelay units comprises one or more amplifiers.
 11. The cable as describedin claim 10, wherein each analog delay unit comprises: first and secondamplifiers having a common input, which is the input of the analog delayunit, and their outputs being summed to generate the output of theanalog delay unit; the first amplifier having a gain of (1.0−Δ), and adelay equal to a predetermined delay value; and the second amplifierhaving a gain of Δ, and substantially the same delay as the firstamplifier.
 12. The cable as described in claim 11, wherein the firstamplifier is a follower stage, and the second amplifier has a shuntcapacitor for setting the gain of Δ.
 13. A boost device for connecting atransmitting data source device to a receiving data sink device, thetransmitting data source device sending differential data signals intothe boost device, the boost device boosting at least one of thedifferential data signals, the boost device comprising: an input circuitfor receiving a raw differential signal from the data source device andoutputting a recovered signal; a deskew circuit with first adjustableparameters for processing the recovered signal into a deskewed signal;an equalizer circuit with second adjustable parameters for processingthe deskewed signal into an equalized signal; an output circuit foramplifying the equalized signal into a boosted signal and sending theboosted signal to the data sink device; and a parameter memory forretaining the first and second adjustable parameter, and a control inputfor accessing the parameter memory.
 14. A method for a sendingdifferential signals from a transmitting data source device to areceiving data sink device through a cable that includes a boost devicefor boosting at least one of the differential signals, comprising thesteps of: receiving a raw differential signal from the data sourcedevice in an input circuit of the boost device and outputting arecovered signal; processing the recovered signal in a deskew circuitwith first adjustable parameters into a deskewed signal; processing thedeskewed signal in an equalizer circuit with second adjustableparameters into an equalized signal; amplifying the equalized signal inan output circuit into a boosted signal; and sending the boosted signalto the data sink device.
 15. The method as described in claim 14,further including the steps of: adjusting the first and secondadjustable parameters; and storing the first and second adjustableparameters in parameter memory.
 16. The method as described in claim 15,further including a step of accessing the parameter memory from acontrol input.
 17. The method as described in claim 14, wherein the stepof processing the recovered signal includes the step of adjusting anexisting time skew of two polarities of the differential signal bychanging the first adjustable parameters.
 18. The method as described inclaim 17, wherein the step of processing the deskewed signal includesthe step of adjusting a frequency response of the deskewed signal bychanging the second adjustable parameters.
 19. The method as describedin claim 17, wherein the step of changing the first adjustableparameters comprises the steps of: arranging a number of delay unitssequentially; selecting a composite delay resulting from number of delayunits; and inserting the composite delay into the polarities of thedifferential signal.
 20. The method as described in claim 19, whereinthe step of inserting the composite delay includes inserting thecomposite delay into one or the other polarity of the differentialsignal.